if_awg: Add missing awg_poll() prototype

[ivy]

Summary

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
Sponsored by: https://www.patreon.com/bsdivy

[glebius]

I wonder if anybody have actually tested that old mode on this relatively new driver. I wonder if anybody in the world runs with the device polling today at all.

[ivy]

since ARMADAXP is a Marvell SoC and if_awg is an Allwinner IC, it's quite possible this combination has never been used by anyone. but, this does currently break the build, and this fix is simpler than either removing DEVICE_POLLING from ARMADAXP or removing polling support from awg.