staging: dgap: fix error path

The code in dgap_stop() is almost a duplicate of the code that will be
executed on pci_unregister_driver(). So the error code was stopping and
unregistering everything twice.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>