USB: dwc3_otg: Move to A_IDLE state if unable to start host

As part of ID clear, OTG driver tries to start host mode. This may
fail if regulator driver is not ready and returns -EPROBE_DEFER
for VBUS regulator_get. Instead of moving to UNDEFINED state, move
OTG state-machine to A_IDLE so that it can retry start_host later
when notified again as part of regulator driver's initialization.

CRs-Fixed: 513074
Change-Id: I1711bfaa0c7ceed01d596f5a10c8d25a8d3931e0
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>