[MTD] SSFDC must depend on BLOCK

This patch fixes the following compile error with
CONFIG_SSFDC=m, CONFIG_BLOCK=n:

<--  snip  -->

...
  CC [M]  drivers/mtd/mtd_blkdevs.o
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mtd/mtd_blkdevs.c:40: warning: ‘struct request’ declared inside parameter list
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mtd/mtd_blkdevs.c:40: warning: its scope is only this definition or declaration, which is probably not what you want
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mtd/mtd_blkdevs.c: In function ‘do_blktrans_request’:
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/mtd/mtd_blkdevs.c:45: error: dereferencing pointer to incomplete type
...
make[3]: *** [drivers/mtd/mtd_blkdevs.o] Error 1

<--  snip  -->

Bug report by Jesper Juhl.

This patch also removes a pointless "default n" from the SSFDC option.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>