Staging: iio: iio_simple_dummy: Remove explicit NULL comparison

This patch removes explicit NULL comparison and writes it in its
simpler form. Done with coccinelle:

@replace_rule@
expression e;
@@

-e == NULL
+ !e

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>