brcmf: Fix null pointer exception in bcdc_hdrpull

In fwsignal.c: brcmf_fws_commit_skb()
...
if (rc < 0) {
  entry->transit_count--;
    if (entry->suppressed)
      entry->suppr_transit_count--;
      (void)brcmf_proto_hdrpull(fws->drvr, false, skb, NULL);
                                                     ^^^^^^^
    goto rollback;
}
...

The call to hdrpull will trigger a null pointer exception
unless a null check is made in the method implementation.

Signed-off-by: Per Forlin <per.forlin@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>