net/ethernet/chelsio/cxgb/cxgb2: Use module_pci_driver to register driver

Removing some boilerplate by using module_pci_driver instead of calling
register and unregister in the otherwise empty init/exit functions.

The name of the pci_driver struct had to be changed in order to prevent
a build failure.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>