drivers: can: mcan: assume can_send() always provides callback

Given the recent change to the CAN controller can_send() API wrapper,
assume that a callback is always provided at the driver level.

This saves approximately 500 bytes of SRAM on the lpcxpressso55s16 board
when running in default configuration.

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