ixgbe: combine ixgbe_add_rx_frag and ixgbe_can_reuse_page

This patch combines ixgbe_add_rx_frag and ixgbe_can_reuse_page into a
single function. The main motivation behind this is to make better use of
the values so that we don't have to load them from memory and into
registers twice.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>