This patch renames mcp251xfd_osc_invalid() to mcp251xfd_reg_invalid(),
as it will be used for other registers than the "osc" register in a
later patch.

This patch also moves this function to more towards the beginning of
the file, to be available for other functions, too.

Link: https://lore.kernel.org/all/20220207131047.282110-2-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>