mtd: mtd in mtd_release is unused without CONFIG_MTD_CHAR

drivers/mtd/mtdcore.c: In function 'mtd_release':
drivers/mtd/mtdcore.c:51: warning: unused variable 'mtd'

[akpm: make it actually build]
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>