sf: Fix NULL pointer exception for flashes without lock methods

commit c3c016cf "sf: Add SPI NOR protection mechanism" introduced
flash_lock()/flash_unlock()/flash_is_locked() methods for SPI flash,
but not every flash driver supplies these. We should test these
methods against NULL before actually calling them.
Signed-off-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NFabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: NJagan Teki <jteki@openedev.com>

drivers/mtd/spi/sf_ops.c

@@ -268,9 +268,12 @@ int spi_flash_cmd_erase_ops(struct spi_flash *flash, u32 offset, size_t len)
 		return -1;
 	}
 
-	if (flash->flash_is_locked(flash, offset, len) > 0) {
-		printf("offset 0x%x is protected and cannot be erased\n", offset);
-		return -EINVAL;
+	if (flash->flash_is_locked) {
+		if (flash->flash_is_locked(flash, offset, len) > 0) {
+			printf("offset 0x%x is protected and cannot be erased\n",
+			       offset);
+			return -EINVAL;
+		}
 	}
 
 	cmd[0] = flash->erase_cmd;
@@ -315,9 +318,12 @@ int spi_flash_cmd_write_ops(struct spi_flash *flash, u32 offset,
 
 	page_size = flash->page_size;
 
-	if (flash->flash_is_locked(flash, offset, len) > 0) {
-		printf("offset 0x%x is protected and cannot be written\n", offset);
-		return -EINVAL;
+	if (flash->flash_is_locked) {
+		if (flash->flash_is_locked(flash, offset, len) > 0) {
+			printf("offset 0x%x is protected and cannot be written\n",
+			       offset);
+			return -EINVAL;
+		}
 	}
 
 	cmd[0] = flash->write_cmd;

include/spi_flash.h

@@ -237,7 +237,7 @@ static inline int spi_flash_erase(struct spi_flash *flash, u32 offset,
 static inline int spi_flash_protect(struct spi_flash *flash, u32 ofs, u32 len,
 					bool prot)
 {
-	if (!flash->flash_lock)
+	if (!flash->flash_lock || !flash->flash_unlock)
 		return -EOPNOTSUPP;
 
 	if (prot)