drivers: staging: rtl8723au: remove unneeded null test

null test on pnetwork removed, because the iterator variable
list_for_each_entry_safe cannot be null.

This commit fixes the following error reported by coccinelle:

drivers/staging/rtl8723au/core/rtw_mlme.c:1621:7-15: ERROR: iterator
variable bound on line 1620 cannot be NULL

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>