staging: rtl8712u: Fix suspend and resume callbacks

When system resume and the rtl8712u dongle didn't lose its power, it
doesn't work quite right:
[  146.844241] r8712u 2-3:1.0 wlp0s4f1u3: Unable to resume
...
[  154.029573] r8712u 2-3:1.0 wlp0s4f1u3: In r8711_wx_set_scan: bDriverStopped=1

The !netif_running() check in PM callbacks aren't correct because
rtl8712u doesn't support runtime suspend, so the when the PM ops get
called, it's system suspend or system resume, hence we want to "stop
everthing" or "resume everything".

The NULL check on pnetdev is also redundant, the only place pnetdev can
become NULL is the disconnect callback, which is mutually exclusive to
PM callbacks.

This can make rtl8712u starts working after system resume.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>