cxgbe(4): Fix an assertion that is not valid during attach.

Firmware access from t4_attach takes place without any synchronization.
The driver should not panic (debug kernels) if something goes wrong in
early communication with the firmware. It should still load so that
it's possible to poke around with cxgbetool.

Sponsored by: Chelsio Communications

(cherry picked from commit 4a4e9c516cfc54181264c92276301a45ea4680d4)