drivers: dac: dac_ad569x: Add bit shift for 14/12-bit variants

The 14/12-bit variants (AD5692R/AD5691R) require the data to be in the
upper bits of the two byte data field of the write command. See table 12
and the associated footnotes in the AD5693R/AD5692R/AD5691R/AD5693
datasheet.

Signed-off-by: Brett Witherspoon <brett@witherspoon.engineering>