i40e: clean up debug_read_register

There were some additional spaces and strange (double swapping) logic
in this function that I started looking at because sparse was warning.

This fixes the sparse warning and fixes up the other issues.

Change-ID: I72a91a4197cd45921602649040e6bd25e5f17c0a
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>