can: c_can: Set reserved bit in IFx_MASK2 to 1 on write

According to C_CAN documentation, the reserved bit in IFx_MASK2 register is
fixed 1.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>