staging: silicom: fix a comparing proc_dir_entry pointer against 0

we should be using the NULL macro, not 0 to compare against
a pointer value, and also remove braces around the single
if conditional after the create_proc_entry

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>