mwifiex: Make mwifiex_dbg a function, reduce object size

The mwifiex_dbg macro has two tests that could be consolidated
into a function reducing overall object size ~10KB (~4%).

So convert the macro into a function.

$ size drivers/net/wireless/mwifiex/built-in.o* (x86-64 defconfig)
   text	   data	    bss	    dec	    hex	filename
 233102	   8628	   4809	 246539	  3c30b	drivers/net/wireless/mwifiex/built-in.o.new
 243949	   8628	   4809	 257386	  3ed6a	drivers/net/wireless/mwifiex/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>