wg: change module name to if_wg

Other virtual interface drivers (e.g. if_gif, if_stf, if_ovpn) all start
with if_. The wireguard file is also named if_wg, but the module name
was 'wg'.

Fix this inconsistency.

Reported by: Christian McDonald <cmcdonald@netgate.com>
Reviewed by: zlei, kevans
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39853