thunderbolt: Do not try to read UID if DROM offset is read as 0

At least Falcon Ridge when in host mode does not have any kind of DROM
available and reading DROM offset returns 0 for these. Do not try to
read DROM any further in that case.
Signed-off-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: NYehezkel Bernat <yehezkel.bernat@intel.com>
Reviewed-by: NMichael Jamet <michael.jamet@intel.com>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NAndreas Noever <andreas.noever@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/eeprom.c

@@ -276,6 +276,9 @@ int tb_drom_read_uid_only(struct tb_switch *sw, u64 *uid)
 	if (res)
 		return res;
 
+	if (drom_offset == 0)
+		return -ENODEV;
+
 	/* read uid */
 	res = tb_eeprom_read_n(sw, drom_offset, data, 9);
 	if (res)