can: c_can: remove unused variable struct c_can_priv::rxmasked

The member rxmasked of struct c_can_priv is initialized by
c_can_chip_config(), but's it's never used, so remove it.

Link: https://lore.kernel.org/r/20210509124309.30024-2-dariobin@libero.it


Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>