can: etas_es58x: add es58x_free_netdevs() to factorize code

Both es58x_probe() and es58x_disconnect() use a similar code snippet
to release the netdev resources. Factorize it in an helper function
named es58x_free_netdevs().

Link: https://lore.kernel.org/r/20210628155420.1176217-5-mailhol.vincent@wanadoo.fr


Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>