ath9k_hw: fix more bitfield related endian issues

A few LNA control related flags were also specified as a bitfields, however
for some strange reason they were written in big-endian order this time.
Fix this by using flags instead.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>