net/sched: flower: Helper function for vlan ethtype checks

There are somewhat repetitive ethertype checks in fl_set_key. Refactor
them into is_vlan_key helper function.

To make the changes clearer, avoid touching identation levels. This is
the job for the next patch in the series.

Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>