if_awg: Add missing awg_poll() prototype

The function awg_poll() was missing a prototype, which causes the build
to fail if DEVICE_POLLING is enabled, which it is in the ARMADAXP config.

MFC after: 2 weeks
Reviewed by: tuexen, mmel, adrian
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56651