drivers/can/rcar: Fix setting bit timing

Fix issue #45169.

With this CAN controller, changing bit timing has to be done in controller
reset mode, resetting some registers to their default values.
TCR register, that is enabling loopback mode is one of these.
Because of this reset, the controller switch back from loopback
to normal mode without the test suite being notified, preventing
receiving sent frames afterwards.

To fix this issue, we are now storing useful registers values before
switching to reset mode and restoring these values in halt mode
before going back in operation mode.

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>