mfd: Clarify twl4030 return value for read and write

We should be checking if all the messages were tranferred. If not, then we
should propagate the i2c core error code or EIO.
Currently we return success (0) even if none of messages were transferred
successfully.

Signed-off-by: Amit Kucheria <amit.kucheria@verdurent.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>