ath9k_hw: Optimize ath9k_hw_ar9287_set_board_values

Rather than doing a series of RMWs, calculate the
value to be written to the register in question and
do a single REGWRITE. This improves bringup time.

This depends on the analog_shiftreg configuration option,
which is currently buggy. For AP mode, a delay of 100us
has to be the default. For station mode, this knob has to
be enabled on a per-case basis, though it is a little
unclear on when to enable a delay. This can be fixed later though.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>