drivers/net/mlx4: Use %pV, pr_<level>, printk_once

Remove near duplication of format string constants by using the newly
introduced vsprintf extention %pV to reduce text by 20k or so.

$ size drivers/net/mlx4/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 161367	   1866	  48784	 212017	  33c31	drivers/net/mlx4/built-in.o
 142621	   1866	  46248	 190735	  2e90f	drivers/net/mlx4/built-in.o.new

Use printk_once as appropriate.
Convert printks to pr_<level>, some bare printks now use pr_cont.
Remove now unused #define PFX.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>