can: mcp251x: Allow tuning SPI mode and limit maximal SPI speed

Patch allow to use different mode settings for SPI (MODE3 for example)
and limit maximal speed according to IC datasheet.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>