mtd: maps: sc520cdp: fix warnings

mtd: maps: sc520cdp: fix warnings

On m86k, and maybe a few other architectures, we get this kind of
warning, due to misuse of volatile:

   drivers/mtd/maps/sc520cdp.c: In function 'sc520cdp_setup_par':
>> drivers/mtd/maps/sc520cdp.c:223:2: warning: passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type [enabled by default]
   arch/m68k/include/asm/raw_io.h:22:13: note: expected 'void *' but argument is of type 'volatile long unsigned int *'

Rather than annotating the variable declaration, let's just use the
proper accessors, which add the 'volatile' qualifier to the operation.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>