qlnx: fix panic at startup

While attaching the qlnx driver we can end up trying to call ifmedia functions
before those are set up (through ether_ifattach() -> rtnl_handle_ifattach()
-> dump_iface() -> get_operstate_ether() -> ifmedia_ioctl()). Postpone the
ether_ifattach() until after the ifmedia setup.

Reviewed by: markj,zlei
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D50819

(cherry picked from commit 4f7473d18a62077a218ac00817bae09a95b8cbcb)