if_vlan_var.h: fix value of DOT1Q_VID_MAX

This should be 4094 (0xFFE), not 4095 (0xFFF), since 0xFFF is not a
valid VID.

Fixes: c59492025073 ("sys/net: add DOT1Q_VID_{MIN,MAX}")
MFC after: 3 days
Reviewed by: kevans, kp, adrian
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50576