can: m_can: make m_can_start and m_can_stop symmetric

This moves clocks gating outside of the m_can_stop function as the
m_can_start function does not (and cannot, at least in current
implementation) ungate clocks. This way, both functions can now be used
symmetrically.

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