drivers: can: nrf: fix invalid pointer leading to undef behavior

The can_nrf device driver incorrectly passes its own device
driver pointer to a call to clock_control_get_rate() to get the
rate of the auxpll. The actual device driver which should be passed
to clock_control_get_rate() is the auxpll.

Without this fix, the call jumps to the can_nrf_api and returns
garbage (unfortunately it does not hardfault, which is why this
issue has not been discovered yet).

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>