sfxge(4): remove Falcon-specific concurrency check

Falcon support has been withdrawn from libefx, however, there is still
an obsolete Falcon-specific assertion that efx_mac_stats_upload()
and efx_port_poll() aren't concurrent. To be consistent with an overall
Falcon support revocation it's desirable to remove it.

Fix debug build invalid assertion failure.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18130