USB: msm_otg: Host: Allow XO shutdown only if no session is active

Currently msm_otg driver allows VDD minimization during host bus
suspend and depends on MPM for USB remote wakeup on platforms,
where D+ and D- lines are routed to MPM. As part of VDD minimization,
PHY will be put into retention state in which pull down resistors
are disabled. This scenario could cause issues without HW rework of
connecting pull down resistors on data lines. Hence don't allow
XO shutdown during host bus suspend if session is active.

Change-Id: Ic7274072cc4f266574cf45030bcadc469091c333
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>