can: m_can: change comparison to bitshift when dealing with a mask

Due to a typo, the mask was destroyed by a comparison instead of a bit
shift.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>