ASoC: Fix wrong define for AD1836_ADC_WORD_OFFSET

According to the datasheet:
The BIT[5:4] of ADC Control Register 2 is to control the word width.
        00 = 25 Bits
        01 = 20 Bits
        10 = 16 Bits
        11 = Invalid

Thus, the AD1836_ADC_WORD_OFFSET should be defined as 4.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org