xhci: Refactor bus suspend state into a struct.

There are several variables in the xhci_hcd structure that are related to
bus suspend and resume state.  There are a couple different port status
arrays that are accessed by port index.  Move those variables into a
separate structure, xhci_bus_state.  Stash that structure in xhci_hcd.

When we have two roothhubs that can be suspended and resumed separately,
we can have two xhci_bus_states, and index into the port arrays in each
structure with the fake roothub port index (not the real hardware port
index).

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