usb: phy: qusb: Rely on regulator_enable() for ref_counting

USB PHY regulators are enabled either by USB core driver for
regular USB operation or by Charger driver before performing
charger detection. To prevent race conditions between these
two, PHY driver currently maintains reference counting in
qusb_phy_enable_power().
Get rid of this reference counting in the driver as
regulator framework already takes care of same.

Change-Id: If0cdb43fe103c81cd94aeb6ff77bf982282f1164
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>