can: mscan: simplify clock enable/disable

All the NULL checks are pointless, clk_*() routines already deal with
NULL just fine.

Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Link: https://lore.kernel.org/r/1594972875-27631-1-git-send-email-zhangchangzhong@huawei.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>