drivers: can: sam0: fix clock configuration for SAM0 series

Fixed a bug where unconfigured clocks were connected to the can
interface in the device tree for SAM0, causing the interface to work
incorrectly. Fixed by adding the correct index when calling GENCTRL.
Also, the default divider has been reduced to 6 to allow setting
the bitrate to 500 kbps.

Tested on a canopennode sample on a board with an ATSAMC21E18A
microcontroller.

Signed-off-by: Vitaliy Livnov <vitaliy.livnov@devkit.agency>