iio: event_monitor: report unsupported events

This makes the event monitor bail out with a helpful error
message if a device does not support events, as a related
fix to iio core now makes it return -ENODEV properly.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>