ath5k: cleanup ath5k_hw struct

ah_gpios array isn't used, and ah_current_channel can be a pointer
instead of an embedded struct.  Removing these and some other
write-only variables, and moving some things around for better
packing and cache utilization saves 116 bytes.

   text	   data	    bss	    dec	    hex	filename
 121762	    472	     64	 122298	  1ddba	ath5k_before.ko
 121646	    472	     64	 122182	  1dd46	ath5k.ko

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