i2c-designware: i2c_dw_xfer_msg: Fix i2c_msg search bug

In case a work-in-progress i2c_msg has more bytes to be written, we
need to set STATUS_WRITE_IN_PROGRESS and exit from the msg_write_idx-
searching loop.  Otherwise, we will overtake the current msg_write_idx
without waiting for its transmission to be processed.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>