i40e: consolidate maximum frame size calculation for vsi

Introduce new helper function to calculate max frame size for validating
and setting of vsi frame size. This is used while configuring vsi,
changing the MTU and attaching an XDP program to the vsi.

This is in preparation of the legacy rx and multi-buffer changes to be
introduced in later patches.

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Tested-by: Chandan Kumar Rout <chandanx.rout@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>