ath5k: preserve higher order bits when setting mac address

In some cases we would like to set the mac address without changing
the operating mode.  However, Atheros cards store PCU data in the high
16 bits of the mac address register.  Change ath5k_hw_set_lladdr() to
not clobber the PCU settings.

Changes-licensed-under: ISC
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>