drivers: can: Extend CAN API for bus-state management.

This commit extends the CAN API with the following functions:
- can_get_state
- can_recover
- can_register_state_change_isr

This functions can be used to get the error-counters and the state
of the CAN controller. The recover function can be used to recover
from bus-off state when automatic recovery is disabled.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>