tools: iio: Remove explicit NULL comparison

Remove explicit NULL comparison and write it in its simpler form as
recommended by checkpatch.pl.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --cc tools/iio/generic_buffer.c
Simple merge

diff --cc tools/iio/iio_utils.c
Simple merge

diff --cc tools/iio/lsiio.c
Simple merge