can: flex_can: Correct the checking for frame length in flexcan_start_xmit()

The flexcan_start_xmit() function compares the frame length with data
register length to write frame content into data[0] and data[1]
register. Data register length is 4 bytes and frame maximum length is 8
bytes.

Fix the check that compares frame length with 3. Because the register
length is 4.

Signed-off-by: Luu An Phu <phu.luuan@nxp.com>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>