thunderbolt: Mask ring interrupt properly when polling starts

When ring enters polling mode we are expected to mask the ring interrupt
before the callback is called. However, the current code actually
unmasks it probably because of a copy-paste mistake.

Mask the interrupt properly from now on.

Fixes: 4ffe722e ("thunderbolt: Add polling mode for rings")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>