sfxge(4): fix efx_ev_qpoll for non-Siena builds

Both Siena and EF10 use the siena_ev_qpoll() implementation, but this
function is not defined in builds without EFSYS_OPT_SIENA.

Remove siena_ev_qpoll and inline it into efx_ev_qpoll to allow it
to be used in non-Siena builds.

Also remove outdated FIXME comment, as EF10 event batching/merging has
been implemented long ago without needing to modify this code.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8939