staging: gasket: core: hold reference on device while in use

Hold a reference on the struct device while a pointer to that
device is in use by gasket.

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