iio: proximity: as3935: fix AS3935_INT mask

AS3935 interrupt mask has been incorrect so valid lightning events
would never trigger an buffer event. Also noise interrupt should be
BIT(0).

Fixes: 24ddb0e4 ("iio: Add AS3935 lightning sensor support")
CC: stable@vger.kernel.org
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>