ASoC: wm8960: remove 'dres' field from platform data structure

The 'dres' field (discharge resistance for headphone outputs) is no longer
used in the driver, so remove it.

It was used in the original version of the driver when entering standby
from off, but we stopped using it when we switched from having a single
startup sequence to having separate cap and capless sequences.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>