mfd: Correct WM8350 I2C return code usage

The vendor BSP used for the WM8350 development provided an I2C driver
which incorrectly returned zero on succesful sends rather than the
number of transmitted bytes, an error which was then propagated into the
WM8350 I2C accessors.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>