can: mcp251xfd: Add support for internal loopback mode

MCP251xFD supports internal loopback mode which can be used to verify CAN
functionality in the absence of a real CAN device.

Link: https://lore.kernel.org/r/20201201054019.11012-1-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[mkl: mcp251xfd_get_normal_mode(): move CAN_CTRLMODE_LOOPBACK check to front]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>