can: flexcan: flexcan_get_berr_counter(): switch on clocks before accessing ecr register

The funcion flexcan_get_berr_counter() may be called from userspace even if the
interface is down, this the clocks are disabled. This patch switches on the
clocks before accessing the ecr register.

Reported-by: Ashutosh Singh <ashuleapyear@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>