[xhci] Use root hub port number to determine slot type

[xhci] Use root hub port number to determine slot type

We currently use the downstream hub's port number to determine the
xHCI slot type for a newly connected USB device.  The downstream hub
port number is irrelevant to the xHCI controller's supported protocols
table: the relevant value is the number of the root hub port through
which the device is attached.

Fix by using the root hub port number instead of the immediate parent
hub's port number.

This bug has not previously been detected since the slot type for the
first N root hub ports will invariably be zero to indicate that these
are USB ports.  For any xHCI controller with a sufficiently large
number of root hub ports, the code would therefore end up happening to
calculate the correct slot type value despite using an incorrect port
number.

Signed-off-by: Michael Brown <mcb30@ipxe.org>