ath5k: use compare_ether_addr on MAC addresses instead of memcmp

Following Felix's recent patchset as an example I have replaced memcmp with
compare_ether_addr.

"Because of the constant size and guaranteed 16 bit alignment, the inline
compare_ether_addr function is much cheaper than calling memcmp."

Signed-off-by: Jonathan Bither <jonbither@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>