efi/apple-properties: Reinstate support for boolean properties

Since commit 4466bf82 ("efi/apple-properties: use
PROPERTY_ENTRY_U8_ARRAY_LEN"), my MacBook Pro issues a -ENODATA error
when trying to assign EFI properties to the discrete GPU:

pci 0000:01:00.0: assigning 56 device properties
pci 0000:01:00.0: error -61 assigning properties

That's because some of the properties have no value.  They're booleans
whose presence can be checked by drivers, e.g. "use-backlight-blanking".

Commit 6e98503d ("efi/apple-properties: Remove redundant attribute
initialization from unmarshal_key_value_pairs()") employed a trick to
store such booleans as u8 arrays (which is the data type used for all
other EFI properties on Macs):  It cleared the property_entry's
"is_array" flag, thereby denoting that the value is stored inline in the
property_entry.

Commit 4466bf82 erroneously removed that trick.  It was probably a
little fragile to begin with.

Reinstate support for boolean properties by explicitly invoking the
PROPERTY_ENTRY_BOOL() initializer for properties with zero-length value.

Fixes: 4466bf82 ("efi/apple-properties: use PROPERTY_ENTRY_U8_ARRAY_LEN")
Cc: <stable@vger.kernel.org>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NLukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/be958bda75331a011d53c696d1deec8dccd06fd2.1609388549.git.lukas@wunner.deSigned-off-by: NArd Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/apple-properties.c

@@ -3,8 +3,9 @@
  * apple-properties.c - EFI device properties on Macs
  * Copyright (C) 2016 Lukas Wunner <lukas@wunner.de>
  *
- * Note, all properties are considered as u8 arrays.
- * To get a value of any of them the caller must use device_property_read_u8_array().
+ * Properties are stored either as:
+ * u8 arrays which can be retrieved with device_property_read_u8_array() or
+ * booleans which can be queried with device_property_present().
  */
 
 #define pr_fmt(fmt) "apple-properties: " fmt
@@ -88,8 +89,12 @@ static void __init unmarshal_key_value_pairs(struct dev_header *dev_header,
 
 		entry_data = ptr + key_len + sizeof(val_len);
 		entry_len = val_len - sizeof(val_len);
-		entry[i] = PROPERTY_ENTRY_U8_ARRAY_LEN(key, entry_data,
-						       entry_len);
+		if (entry_len)
+			entry[i] = PROPERTY_ENTRY_U8_ARRAY_LEN(key, entry_data,
+							       entry_len);
+		else
+			entry[i] = PROPERTY_ENTRY_BOOL(key);
+
 		if (dump_properties) {
 			dev_info(dev, "property: %s\n", key);
 			print_hex_dump(KERN_INFO, pr_fmt(), DUMP_PREFIX_OFFSET,