mac80211: build airo on x86 only

airo requires ISA_DMA_API and
that symbol is only set on some ppc,malta,x86
x86 is probably only platform where that driver is used

fixes buildbot errors on ar71xx,lantiq ...:
airo.ko is missing

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>

SVN-Revision: 49073