drivers: can: change can_tx_callback_t function signature

Change the can_tx_callback_t function signature to use an "int" (not an
uint32_t) for representing transmission errors.

The "error" callback function parameter is functionally equivalent to
the return value from can_send() and thus needs to use the same data
type and needs to be able to hold negative errno values.

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