ifconfig/ifbridge.c: add get_vlan_id()

This is like get_val() but takes an ether_vlanid_t* and ensures the
value is a valid VLAN ID. This avoids redundant comparisons and
casting when parsing VLAN IDs.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D51548

(cherry picked from commit 287a5fdcd3c941ce73705c664b5df4932ba3bad4)