staging: gasket: page table: hold references to device and pci_dev

Hold references to the struct device and the pci_dev for the page table
while the data structures contian pointers to these.

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