Adjust t4_port_init() to work with VF devices.

Specifically, the FW_PORT_CMD may or may not work for a VF (the PF
driver can choose whether or not to permit access to this command),
so don't attempt to fetch port information on a VF if permission is
denied by the PF.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7511