Use a single write of 3 bytes instead of iicdev_writeto() in ads111x.

The iicdev_writeto() function basically does scatter-gather IO by filling
in a pair of iic_msg structs to write the register address then the data
from different locations but with a single bus START/xfer/STOP sequence.
It turns out several low-level i2c controller drivers do not honor the
IIC_NOSTART flag, so the second piece of the write gets a new START on
the bus, and that confuses the ads111x chips which expect a continuous
write of 3 bytes to set a register.

A proper fix for this is to track down all the misbehaving controllers
drivers and fix them. For now this change makes this driver work again.