qmp-phy: Power down PHY durng disconnect to avoid leakage

Currently driver is not powering down QMP PHY if target is booted
up without cable connected. Due to this, 0.3mA leakage current is seen
during standby mode if power measured after bootup. Fix this by
putting QMP PHY in power down state during disconnect.

Change-Id: Idbf7e06ca14649797c50853cf3b9ee182f70749e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>