can: m_can: make custom bittiming fields const

The assigned timing structs will be defined a const anyway, so we can
avoid a few casts by declaring the struct fields as const as well.

Link: https://lore.kernel.org/all/4508fa4e639164b2584c49a065d90c78a91fa568.1636967198.git.matthias.schiffer@ew.tq-group.com


Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>