staging: brcm80211: replaced #ifdef __mips__ sections by W_REG_FLUSH

Code cleanup. A read-after-write construct is present in the code to ensure
write order for certain Broadcom chips. Those chips are: bcm4706, bcm4716,
bcm4717, bcm4718. All these chips contain a MIPS processor. This patch gets
rid of several #ifdef __mips__ sections by defining a new macro in a header
file. This patch does not introduce behavioral changes and is purely meant
for code cleanup. The __mips__ define will be made more specific in a future
patch.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>