can: mcp251xfd: mark some instances of struct mcp251xfd_priv as const

can: mcp251xfd: mark some instances of struct mcp251xfd_priv as const

With the patch 07ff4aed015c ("time/timecounter: Mark 1st argument of
timecounter_cyc2time() as const") some instances of the struct
mcp251xfd_priv can be marked as const. This patch marks these as
const.

Link: https://lore.kernel.org/r/20210813091027.159379-1-mkl@pengutronix.de
Cc: Manivannan Sadhasivam <mani@kernel.org>
Cc: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>