mtd: block2mtd: remove redundant initialization of 'bdev'

Pointer bdev is being initialized however this value is never
read as bdev is assigned an updated value from the returned
call to blkdev_get_by_path.  Remove the redundant assignment.

Cleans up clang warning:
drivers/mtd/devices/block2mtd.c:228:23: warning: Value stored to
'bdev' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>