USB: EHCI: msm: Fix bug in accessing hcd structure during suspend

Host driver's platform_suspend/resume routines are valid only if
LINK is operating in host mode and root-hub has been registered.
Hence, check for 'rh_registered' flag of hcd to process
platform_suspend/resume further.

Change-Id: I5278bd92b30ad5b3a154b5f9f505982097f39ace
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>