drivers: can: mcux: flexcan: assume can_send() always provides callback
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 <24a3f13c22635bab658e39fc5f1e63c575328aae@vestas.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr