staging:iio:ad799x fix typo in ad799x_events[]

This patch fixes a typo in ad799x_events[], which caused the error "Failed to register event set".

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>