staging:iio: Add missing event code extract macros

Add macros for extracting whether the event is for a differential channel and
the second channel number from the event code. These were the only two fields
which did not have such an macro yet.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>