ath9k: work around gcc ICEs

This patch works around an internal compiler error (gcc bug #37014) in
all gcc 4.2 compilers and the gcc 4.3 series up to at least 4.3.1
on at least powerpc and mips.

Many thanks to Andrew Pinski for analyzing the gcc bug.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>