CANCTRL
mcp251x_write_reg(spi, CANCTRL, CANCTRL_REQOP_SLEEP);
mcp251x_write_reg(spi, CANCTRL, CANCTRL_REQOP_CONF);
mcp251x_write_reg(spi, CANCTRL, CANCTRL_REQOP_LOOPBACK);
mcp251x_write_reg(spi, CANCTRL, CANCTRL_REQOP_LISTEN_ONLY);
mcp251x_write_reg(spi, CANCTRL, CANCTRL_REQOP_NORMAL);
ctrl = mcp251x_read_reg(spi, CANCTRL);