can: mcp251xfd: move struct mcp251xfd_tef_ring definition

This patch moves the struct mcp251xfd_tef_ring upwards, so that the union
mcp251xfd_write_reg_buf and struct spi_transfer can be made members of it.

Link: https://lore.kernel.org/r/20201126132144.351154-5-mkl@pengutronix.de


Tested-by: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>