staging: gasket: save struct device for a gasket device

Save the struct device pointer to a gasket device in gasket's metadata,
to facilitate use of standard logging calls and in anticipation of
non-PCI gasket devices in the future.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>