ixgbe: Use out-of-line function for register reads

Register reads are slow, so don't inline them.

Size before:
   text	   data	    bss	    dec	    hex	filename
 226337	   8280	    552	 235169	  396a1	ixgbe.ko

Size after:
   text	   data	    bss	    dec	    hex	filename
 194578	   8280	    552	 203410	  31a92	ixgbe.ko

for about a 14% reduction in text size.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>