driver: net: can: disable clock when it is in enable status

If disable a clock when it is already in disable status, there
will be a warning trace generated. So, it is need to confirm
whether what status the clock is in before disable it.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>