staging: xgifb: main: use readb()/readw() to access iomapped memory

Use readb()/readw() instead of direct pointer access to read I/O mapped
memory and also add __iomem annotation.

The patch eliminates the following sparse warnings:

drivers/staging/xgifb/XGI_main_26.c:2125:35: warning: incorrect type in assignment (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2125:35:    expected char *[addressable] [toplevel] [assigned] mmio_vbase
drivers/staging/xgifb/XGI_main_26.c:2125:35:    got void [noderef] <asn:2>*
drivers/staging/xgifb/XGI_main_26.c:2439:31: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2439:31:    expected void volatile [noderef] <asn:2>*addr
drivers/staging/xgifb/XGI_main_26.c:2439:31:    got char *[addressable] [toplevel] [assigned] mmio_vbase
drivers/staging/xgifb/XGI_main_26.c:2463:31: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2463:31:    expected void volatile [noderef] <asn:2>*addr
drivers/staging/xgifb/XGI_main_26.c:2463:31:    got char *[addressable] [toplevel] [assigned] mmio_vbase

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>