igb: make tx ring map and free functionality non-static

This change makes a minor change to the xmit_frame_ring_adv funcition in that
it moves 2 checks from it into the xmit_frame_adv since the checks were not
ring specific.  In addition it exports the xmit_frame_ring_adv and the
unmap_and_free_tx_resource calls so that they can be used by other code such
as the ethtool loopback testing calls.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>