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.

CRs-fixed: 675563
Change-Id: I891d7a8da6957a10c8c38f39ff024a90c17bde7b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>