LinuxKPI: napi_schedule() requires return value, implement napi_is_scheduled()

A newer version of iwlwifi requires a return value from napi_schedule();
unclear if the function always should have been bool. Add the bool to test
based on the napi_schedule_prep() result.

Also add napi_is_scheduled() for rtw89.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste (previous version)
Differential Revision: https://reviews.freebsd.org/D44591