staging: brcm80211: restrict register access method for bcm47xx

The driver contained conditional code for resolving issue with
dma transaction reordering. This code was conditionalized using
__mips__ macro, but it actually is specific to bcm47xx chips.
This patch replaces it for the more speficic CONFIG_BCM47XX macro.

Tested on BCM63281.

Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Tested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>