ath: Make ath_printk void not int and remove unused struct ath_common *

Changing the return type and removing the unused argument from
ath_printk reduces code size.

Add an __always_unused struct ath_common * to the macros
that call ath_printk to avoid unused variable warnings.

$ size drivers/net/wireless/ath/built-in.o*
   text	   data	    bss	    dec	    hex	filename
1159859	  16235	 212000	1388094	 152e3e	drivers/net/wireless/ath/built-in.o.new
1164175	  16235	 212032	1392442	 153f3a	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>