Staging: brcm80211: disable drivers except for X86 or MIPS platforms

As David points out, the driver is also broken on SPARC, so might
as well just only enable it on platforms where people have reported it
working, instead of trying to list all of the ones where it doesn't
work, as the working platform list is much smaller...

Reported-by: David Miller <davem@davemloft.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Henry Ptasinski <henryp@broadcom.com>
Cc: Brett Rudley <brudley@broadcom.com>
Cc: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>