This patch splits the initialization of the TEF, TX and RX FIFO in the
mcp251xfd_ring_init() function into separate functions. This is a
preparation patch to move the RX FIFO in front of the TX FIFO.

Link: https://lore.kernel.org/all/20220217103826.2299157-3-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>