cramfs: fix MTD dependency

With CONFIG_MTD=m and CONFIG_CRAMFS=y, we now get a link failure:

  fs/cramfs/inode.o: In function `cramfs_mount': inode.c:(.text+0x220): undefined reference to `mount_mtd'
  fs/cramfs/inode.o: In function `cramfs_mtd_fill_super':
  inode.c:(.text+0x6d8): undefined reference to `mtd_point'
  inode.c:(.text+0xae4): undefined reference to `mtd_unpoint'

This adds a more specific Kconfig dependency to avoid the broken
configuration.

Alternatively we could make CRAMFS itself depend on "MTD || !MTD" with a
similar result.

Fixes: 99c18ce5

 ("cramfs: direct memory access support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>