ath5k: Introduce _ath5k_printk to reduce code/text

Macros can be converted to functions to reduce overall object size.

Convert the ATH5K_PRINTK macro to use _ath5k_printk.

Allyesconfig size is reduced ~10%

$ size drivers/net/wireless/ath/ath5k/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 211557	   2032	  40672	 254261	  3e135	drivers/net/wireless/ath/ath5k/built-in.o.new
 235412	   2032	  47296	 284740	  45844	drivers/net/wireless/ath/ath5k/built-in.o.old

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