staging: xillybus: Remember device pointer for use with dev_* functions

This is necessary so that xillybus_core uses the correct device pointer
for PCIe devices in diagnostic message calls (dev_err, dev_warn and dev_info)

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>