USB: OHCI: fix and explain sparse errors

This patch fixes an endian-related error in ohci-hcd (detected by
sparse) and clarifies a comment explaining a peculiar locking
arrangement that sparse warns about.

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