can: m_can: factorize clock gating and ungating

This creates a function to ungate M_CAN clocks and another to gate the
same clocks, then swaps all gating/ungating code with their respective
function.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>