staging: dgnc: unregister pci driver

We may choose to load the module without the hardware present. That will
register the pci driver but since probe will not succeed so
dgnc_NumBoards will be 0. Now if we unload the module then the pci
driver stays registered as dgnc_NumBoards is 0. And if we try to load
the module again it fails with the error:
"Driver 'dgnc' is already registered."

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