i2c-designware: i2c_dw_xfer_msg can be static

i2c_dw_xfer_msg is only called internally so it can be static. It
original was, before the driver split. No idea why it was changed at
that time.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Dirk Brandewie <dirk.brandewie@gmail.com>