usb: host: xhci: move HC_STATE_SUSPENDED check to xhci_suspend()

that check will have to be done by all users
of xhci_suspend() so it sounds a lot better to
move the check to xhci_suspend() in order to
avoid code duplication.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>