drivers: can: provide default callback to can_send() if NULL

Provide a default, internal callback to CAN controller drivers
implementation of can_send() if no callback was provided by the caller.

This allows for simplifying the CAN driver implementations of can_send() as
these no longer need special handling for callback/no callback operation.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
2 files changed