staging: gasket: core: factor out generic device add code from PCI code

Split out generic gasket device add code from the code for adding a PCI
gasket device, in prep for other gasket device types in the future.

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