cxgb4vf: Enable interrupts before we register our network devices

This avoids a race condition where a system that has network devices set up
to be automatically configured and we get the first Port Link Status
message from the firmware on the Asynchronous Firmware Event Queue before
we've enabled interrupts. If that happens, we end up losing the interrupt
and never realizing that the links has actually come up.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>