[MTD] fix nftl_write warning

Building 2.6.18-mm2 issues the following warning if CONFIG_NFTL_RW is not set:

  CC [M]  drivers/mtd/nftlcore.o
drivers/mtd/nftlcore.c:183: warning: 'nftl_write' defined but not used
The following patch only compiles nftl_write if CONFIG_NFTL_RW is set.

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>