can: gs_usb: uniformly use "parent" as variable name for struct gs_usb

To ease readability and maintainability uniformly use the variable
name "parent" for the struct gs_usb in the gs_usb driver.

Link: https://lore.kernel.org/all/20230718-gs_usb-cleanups-v1-4-c3b9154ec605@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>