can: mcp251x: avoid repeated frame bug

The MCP2515 has a silicon bug causing repeated frame transmission, see section
5 of MCP2515 Rev. B Silicon Errata Revision G (March 2007).

Basically, setting TXBnCTRL.TXREQ in either SPI mode (00 or 11) will eventually
cause the bug. The workaround proposed by Microchip is to use mode 00 and send
a RTS command on the SPI bus to initiate the transmission.

Cc: <stable@vger.kernel.org>
Signed-off-by: Benoît Locher <Benoit.Locher@skf.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>