rtlwifi: Optimize RT_TRACE macro use of KBUILD_MODNAME for size

Moving the KBUILD_MODNAME to the format reduces the
overall object size a small amount.

$ size drivers/net/wireless/rtlwifi/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 586904	  55333	 127216	 769453	  bbdad	drivers/net/wireless/rtlwifi/built-in.o.new
 588901	  55333	 127216	 771450	  bc57a	drivers/net/wireless/rtlwifi/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>