ath: Make ath_dbg void not int

The return value is never used so make it void.

Reduces object size a tiny bit.

$ size drivers/net/wireless/ath/built-in.o*
   text	   data	    bss	    dec	    hex	filename
1164175	  16235	 212032	1392442	 153f3a	drivers/net/wireless/ath/built-in.o.new
1164819	  16235	 212032	1393086	 1541be	drivers/net/wireless/ath/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>