mtd: nuc900_nand: NULL dereference in nuc900_nand_enable()

Instead of writing to "nand->reg + REG_FMICSR" we write to "REG_FMICSR"
which is NULL and not a valid register.

Fixes: 8bff82cb ('mtd: add nand support for w90p910 (v2)')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>