can: fix return value from can_get_bittiming()

When trying to set a data bitrate on non CAN FD devices the 'ip' tool
answers with:

	RTNETLINK answers: Unknown error 524

Rename '-ENOTSUPP' to '-EOPNOTSUPP' so that 'ip' answers correctly:

       RTNETLINK answers: Operation not supported

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>