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

Separate code for generic parts of gasket device removal sequence from
the PCI device removal code, in prep for non-PCI devices later.

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