USB: xhci: Remove the arbitrary limit of 15 xHCI ports

Our virtual xHCI device can have as many ports as we like - I've tested
this patch with 31.

Signed-off-by: William Gulland <wgulland@vmware.com>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>