ASoC: wcd9xxx: Account for inconsistency in dce_z during button press

It is observed that dce gnd recalibration value will change
during button press. Currently dce_z is recalibrated only during
first button press. But this would cause issues in further button
detection due to the measurement inconsistancies in dce_z.
Fix this by recalibrating everytime during button press.

Change-Id: I13d9600b20408b1898b205201c3f076e4d093c9e
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>