qusb2: Add missing phy regulator calls in shutdown callback

dwc3 implementation sets up the phy using phy init before
adding xhci platform device. xhci platform driver also sets
up phy using init api. This causes phy ldos to be enabled
twice as phy driver does not turn off regulators in shutdown
callback. While at it, remove legacy code which disables the
phy as phy will be disabled in set_suspend callback.

Change-Id: Ic7320815000681dc0fa1da36db82705f11da8ae5
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>