ath9k_hw: remove a useless function for setting the mac address

ath9k_hw_setmac() only copies the mac address it is called with into
common->macaddr, yet in all call sites, the supplied mac address pointer
is already common->macaddr.

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