can: mcp251xfd: mcp251xfd_get_timestamp(): move to mcp251xfd.h

This is a preparation patch, it moves the mcp251xfd_get_timestamp()
function into the mcp251xfd.h file.

Link: https://lore.kernel.org/r/20210304160328.2752293-5-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>