staging: dgnc: remove pci_unregister_driver

If pci_register_driver() fails then dgnc_NumBoards can never be more
than zero. dgnc_NumBoards is incremented only at the end of a successful
probe. And moreover if the pci driver has failed to register then we
never need to unregister it. Lets just print the warning, perform the
cleanup and exit with the error code.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>