xhci: move the common code to a function to get max ports and port array

There are several functions using same code to get max ports and port array,
this patch moves the common code to a function in order to reuse them easily.

Signed-off-by: Huajun Li <huajun.li.lee@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>