vlan: deduplicate bpf_setpcp() and pf_ieee8021q_setpcp()

These two fuctions were identical, so move them into the common
vlan_set_pcp() function, exposed in the if_vlan_var.h header.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31275