m_can_clk_start() already skip starting the clock when
clock support is disabled, remove the redundant check in
m_can_class_register().

This also solves the imbalance with m_can_clk_stop() that is called
afterward in the same function before the return.
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20240104235723.46931-1-francesco@dolcini.it
[mkl: rebased to net-next/main]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>