cxgbe: use explicit PCI driver structure

The upcoming Hyper-V driver converts the pci_drv element
in struct eth_driver to a union.  When vmbus is added the
pci_drv needs to be explicit. Easier to fix the issue
ahead of time.

This is backwards compatible with previous code.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>