mtd: nuc900_nand: read correct SMISR register

The nuc900_nand driver has always passed an incorrect register
address in its nuc900_check_rb() function, which cannot possibly
work, and in some configurations gives us a build warning:

drivers/mtd/nand/nuc900_nand.c: In function 'nuc900_check_rb':
drivers/mtd/nand/nuc900_nand.c:27:23: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast [-Wint-conversion]
 #define REG_SMISR     0xac
drivers/mtd/nand/nuc900_nand.c:118:20: note: in expansion of macro 'REG_SMISR'
  val = __raw_readl(REG_SMISR);

This makes sure we actually read from the register rather than
from (void *)0x000000ac in user space.

I suspect nobody noticed this before because the nuc900_nand_devready()
function never gets called, or nobody uses this driver on an upstream
kernel. Possibly even both.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>