i2c-davinci: Fix TX setup for more SoCs

This patch is an improvement to 4bba0fd8
which got to mainline a little early.

Sudhakar Rajashekhara explains that at least OMAP-L138 requires MDR mode
settings before DXR for correct behaviour, so load MDR first with
STT cleared and later load again with STT set.

Tested on DM355 connected to Techwell TW2836 and Wolfson WM8985

Signed-off-by: Jon Povey <jon.povey@racelogic.co.uk>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>
Tested-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>