cros_ec: i2c: Group i2c write / read into single transaction

Replace dm_i2c_write() / dm_i2c_read() with transaction using
struct i2c_msg[2] in order to allow for i2c controller to detect
write/read cycle to emit a repeated start condition.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: u-boot@lists.denx.de
Acked-by: Simon Glass <sjg@chromium.org>
Tested on snow:
Tested-by: Simon Glass <sjg@chromium.org>