ath9k: enable serialize_regmode for non-PCIE AR9287

https://bugzilla.kernel.org/show_bug.cgi?id=42903

Based on the work of <fynivx@gmail.com>

Signed-off-by: Panayiotis Karabassis <panayk@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>