can: fix unchecked return from clock_control_get_rate
can: fix unchecked return from clock_control_get_rate

Inside can_stm32_runtime_configure() result of
clock_control_get_rate() is not checked which might result that
function can return error and that error can not be handled

Coverity-CID: 190926
Fixes: #13886
Signed-off-by: Maksim Masalski <4eb3679da23cf41c68efaea5c81c9e87b8df5f95@gmail.com>

Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr