can: m_can: m_can_class_unregister(): remove erroneous m_can_clk_stop()

In m_can_class_register() the clock is started, but stopped on exit. When
calling m_can_class_unregister(), the clock is stopped a second time.

This patch removes the erroneous m_can_clk_stop() in  m_can_class_unregister().

Fixes: f524f829

 ("can: m_can: Create a m_can platform framework")
Cc: Dan Murphy <dmurphy@ti.com>
Cc: Sriram Dash <sriram.dash@samsung.com>
Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20201215103238.524029-2-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>