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

commit 2bd3bc4e8472424f1a6009825397639a8968920a upstream.

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

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>