usb-phy-qusb: Preserve the reset value of powerdown register

Currently we are powering down PHY by setting the POWER_DOWN
bit of QUSB2PHY_QUSB2PHY_PORT_POWERDOWN PHY register. This is
causing leakage when device is booted without USB cable connected.
As per HPG, we have to retain the reset value of powerdown register
by not clearing RETEN_EN and FREEZIO_N bits.
To fix this, preserve the reset value of powerdown register while
setting the POWER_DOWN bit of QUSB2PHY_QUSB2PHY_PORT_POWERDOWN PHY
register to avoid leakage.

Change-Id: I6dfaa7d3fb4e19b36e11fea829e45d1bfebbce02
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>