drivers: can: mcux: flexcan: 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 frdm_k64f board, while on
the mimxrt1024_evk board the saving is approximately 2.5 kbytes of SRAM,
both when running in default configuration.

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