uas: Allow uas_use_uas_driver to return usb-storage flags

uas_use_uas_driver may set some US_FL_foo flags during detection, currently
these are stored in a local variable and then throw away, but these may be
of interest to the caller, so add an extra parameter to (optionally) return
the detected flags, and use this in the uas driver.

Cc: stable@vger.kernel.org # 3.16
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>