staging: iio: fix trigger handler of iio_simple_dummy driver

accessing first and last channel fails:
fakedata[0] is never accessed, out-of-bound access for last channel

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>