ASoC: msm8960: properly protect external mclk control function

Both audio path and MBHC accessory detection logic need master clock
enabled along with bandgap and clock block of CODEC. Clock control
is managed by the machine driver. Calls to clock control function from
audio and MBHC detection paths can be nested. As a result, reference
counter of master clock is incremen/decrement out of order and cause
master clock not enabled when audio path is enabled. Without master clock
CODEC will not consume data and this leads to SLIMBUS overflow error.

Change-Id: I2cabb2d6db4c0129716489672e34a49cc21cea08
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>