sfxge: defer ether_ifattach to when ifmedia_init is done

Otherwise the ifmedia subsystem may call unguarded NULL function
pointers. Same issue that was fixed for cxgb(4) in f2daf8995.

Also see: https://github.com/opnsense/src/issues/228

(cherry picked from commit 0482974b2c021a8abbc044d3e2b1240250f791e7)