staging: brcm80211: fixed build issue for big endian platforms

Driver now builds for big endian mips platform, possibly also for other
big endian platforms. A change was made to the R_REG and W_REG macro's.
These macro's perform an xor (^) operation for endianess swap purposes.
Gcc complained because an xor operation is not allowed on a pointer type.
Fixed this by casting the pointer to an unsigned long.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>