can: mcp251x: add mcp251x_write_2regs() and make use of it

This patch introduces the function mcp251x_write_2regs() to write two
registers with one SPI transfer and converts the disabling of pending
interrupts in mcp251x_stop() to it.

Signed-off-by: Timo Schlüßler <schluessler@krause.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>