usb: ehci-msm: Don't call ehci_suspend during system suspend

EHCI platform glue driver can call ehci_suspend to update
wakeup flags of root hub during system suspend. This requires
bringing hardware out of low power state to access its registers.
Since MSM USB OTG driver uses pm_relax APIs in its runtime_pm
hooks, resuming the OTG device during pm_resume would result in
system suspend abort. Additionally MSM USB host is always
wakeup capable, therefore, don't call ehci_suspend to avoid
bringing h/w out of LPM during pm_suspend.

Change-Id: I5578e76d073b3e23dbcff71251f2594122cb4199
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>