mtd: sst25l, fix lock imbalance

Add an omitted unlock to one sst25l_erase fail path.

Signed-off-by: Jiri Slaby <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>
Signed-off-by: David Woodhouse <[e-mail]>

drivers/mtd/devices/sst25l.c

···
 	mutex_lock(&flash->lock);
 
 	err = sst25l_wait_till_ready(flash);
-	if (err)
+	if (err) {
+		mutex_unlock(&flash->lock);
 		return err;
+	}
 
 	while (addr < end) {
 		err = sst25l_erase_sector(flash, addr);