ixgbe: Resolve warnings for -Wimplicit-fallthrough

This patch adds/changes fall through comments to address new warnings
produced by gcc 7.

Fixed formatting on a couple of comments in the function.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>