xhci: Rename variables and reduce register reads.

The xhci_bus_suspend() and xhci_bus_resume() functions are a bit hard to
read, because they have an ambiguously named variable "port".  Rename it
to "port_index".  Introduce a new temporary variable, "max_ports" that
holds the maximum number of roothub ports the host controller supports.
This will reduce the number of register reads, and make it easy to change
the maximum number of ports when there are two roothubs.

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