Staging: iio: Simplify NULL pointer condition

Replace NULL comparrison with its shorter 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>