tools:iio:iio_event_monitor: refactor events output

Refactor the code in print_event() to reduce code duplication and better
reflect that the type is output unconditionally, as well as cascade the
dependency of the diff-channel. Saves a few lines of code, as well.

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