Staging: iio: meter: Remove explicit NULL comparison

This patch removes explicit NULL comparison and replaces it with
its shorter form. Detected 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>