staging/usbip: userspace - fix usbipd SIGSEGV from refresh_exported_devices()

refresh_exported_devices() doesn't check udev_device_new_from_syspath()
return value and passed in null dev to udev_device_get_driver() resulting
in a segmentation fault. Change it to check for null return value from
both udev_device_new_from_syspath() and udev_device_get_driver().

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>