can: mcp251xfd: fix compilation

Either switching to CAN_DEVICE_DT_INST_DEFINE with [1] missed
updating mcp251xfd or missed in merge. Fix using function
pointer for init in mcp251xfd.

[1]: https://github.com/zephyrproject-rtos/zephyr/pull/62925



Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>