i2c-omap: Add support for I2C_M_STOP message flag

Generate a stop condition after each message marked with I2C_M_STOP.

[JD: Add I2C_FUNC_PROTOCOL_MANGLING.]

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>