ASoC: wcd9xxx-mbhc: Avoid flipping current source setting

In some cases, the current source switch on codec is disabled and
enabled again where it could have been left in enabled state. This
flipping of current source setting is causing a pop on the headphones.
Add fix to avoid this unnecessary flipping of current source switch.

Change-Id: I36055218230034b14a2c9536e176d1c6f8a1881e
CRs-fixed: 675563
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>