UBI: fix warnings

drivers/mtd/ubi/cdev.c: In function ‘vol_cdev_read’:
drivers/mtd/ubi/cdev.c:187: warning: unused variable ‘vol_id’
CC [M]  drivers/mtd/ubi/kapi.o
drivers/mtd/ubi/kapi.c: In function ‘ubi_leb_erase’:
drivers/mtd/ubi/kapi.c:483: warning: unused variable ‘vol_id’
drivers/mtd/ubi/kapi.c: In function ‘ubi_leb_unmap’:
drivers/mtd/ubi/kapi.c:544: warning: unused variable ‘vol_id’
drivers/mtd/ubi/kapi.c: In function ‘ubi_leb_map’:
drivers/mtd/ubi/kapi.c:582: warning: unused variable ‘vol_id’

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/kapi.c b/drivers/mtd/ubi/kapi.c
index 146957c..a70d588 100644
--- a/drivers/mtd/ubi/kapi.c
+++ b/drivers/mtd/ubi/kapi.c
@@ -480,9 +480,9 @@
 {
 	struct ubi_volume *vol = desc->vol;
 	struct ubi_device *ubi = vol->ubi;
-	int err, vol_id = vol->vol_id;
+	int err;
 
-	dbg_msg("erase LEB %d:%d", vol_id, lnum);
+	dbg_msg("erase LEB %d:%d", vol->vol_id, lnum);
 
 	if (desc->mode == UBI_READONLY || vol->vol_type == UBI_STATIC_VOLUME)
 		return -EROFS;
@@ -541,9 +541,8 @@
 {
 	struct ubi_volume *vol = desc->vol;
 	struct ubi_device *ubi = vol->ubi;
-	int vol_id = vol->vol_id;
 
-	dbg_msg("unmap LEB %d:%d", vol_id, lnum);
+	dbg_msg("unmap LEB %d:%d", vol->vol_id, lnum);
 
 	if (desc->mode == UBI_READONLY || vol->vol_type == UBI_STATIC_VOLUME)
 		return -EROFS;
@@ -579,9 +578,8 @@
 {
 	struct ubi_volume *vol = desc->vol;
 	struct ubi_device *ubi = vol->ubi;
-	int vol_id = vol->vol_id;
 
-	dbg_msg("unmap LEB %d:%d", vol_id, lnum);
+	dbg_msg("unmap LEB %d:%d", vol->vol_id, lnum);
 
 	if (desc->mode == UBI_READONLY || vol->vol_type == UBI_STATIC_VOLUME)
 		return -EROFS;