ixgbevf: allow changing MTU when XDP program is attached

xskxceiver attempts to change MTU after attaching XDP program,
ixgbevf rejects the request leading to test being failed.

Support MTU change operation even when XDP program is already attached,
perform the same frame size check as when attaching a program.

Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>