ixgbe: Cleanup DCB logic, whitespace, and comments in ixgbe_ethtool.c

This change address a few whitespace issues in DCB #ifdefs, adds a comment
calling out the DCB specific registers, and nests an if statement inline
with a number of if statements related to flow control.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>