Staging: ion: fix code style warning from NULL comparisons

This patch replaces several instances where a pointer is compared to NULL
(i.e., `ptr == NULL`) with `!ptr`, which is preferred.

Signed-off-by: Quytelda Kahja <quytelda@tamalin.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>