USB: change name of spinlock in hcd.c

This patch (as940 renames hcd_data_lock in hcd.c to hcd_urb_list_lock,
which is more descriptive of the lock's job.  It also introduces a
convenient inline routine for testing whether a particular USB device
is a root hub.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>