Staging: iio: Remove explicit comparison to NULL

This patch simplifies pointer comparison to NULL and makes code
easier to read. Warning found by checkpatch.pl.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>