drivers: can: Fix R-Car driver can_stop behavior

Calling for can_stop() method was not emptying FIFOs
and tx_msgq.
Resetting TX FIFO to empty it.
Emptying tx_msgq with "ENETDOWN" return code.

Fixes #50546

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