sfxge(4): prevent access to the NIC config before probe

NIC config is initialized during NIC probe.

Submitted by:   Mark Spender <mspender at solarflare.com>
Sponsored by:   Solarflare Communications, Inc.
MFC after:      1 week
Differential Revision:  https://reviews.freebsd.org/D18263

sys/dev/sfxge/common/efx_nic.c

@@ -624,6 +624,7 @@ efx_nic_cfg_get(
 	__in		efx_nic_t *enp)
 {
 	EFSYS_ASSERT3U(enp->en_magic, ==, EFX_NIC_MAGIC);
+	EFSYS_ASSERT3U(enp->en_mod_flags, &, EFX_MOD_PROBE);
 
 	return (&(enp->en_nic_cfg));
 }