ixgbevf: Cleanup accounting for space needed at start of xmit_frame

This change cleans up the accounting needed at the start of xmit_frame so
that we can avoid doing too much work to determine how many descriptors we
will need.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>