USB: phy-msm-qusb: Fix race conditions during fast PI/PO

During fast cable connect/disconnect, there is chance that
set_suspend() called as part of USB entering low power mode
and PMIC driver asking to remove pulldowns race each other.
This could cause unclocked access and results in crash. Fix this
issue by using mutex to avoid crash due to the race mentioned.

Change-Id: I1babb545d40b8761774cbdd3a57258ed6bc77398
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>