staging: dgap: dgap_fep5: Do not use 0 for NULL pointer

Do not compare NULL pointer with 0.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>