staging:iio:ad7291 define alert register bits

Define some names for alert register bits and apply them to
ad7291_event_handler().

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>