mac80211: brcmfmac: fix use-after-free & possible NULL pointer dereference

1) Using fwctx variable after brcmf_fw_request_done() was executed meant
   accessing freed memory.
2) Using fwctx->completion for the wait_for_completion_timeout() call
   could reuslt in NULL pointer dereference on fw loading error or if
   brcmf_fw_request_done() was executed quickly enough.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

package/kernel/mac80211/patches/brcm/860-brcmfmac-register-wiphy-s-during-module_init.patch

@@ -58,12 +58,11 @@ Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
  
  	ret = request_firmware_nowait(THIS_MODULE, true, first->path,
  				      fwctx->dev, GFP_KERNEL, fwctx,
-@@ -696,6 +703,9 @@ int brcmf_fw_get_firmwares(struct device
+@@ -696,6 +703,8 @@ int brcmf_fw_get_firmwares(struct device
  	if (ret < 0)
  		brcmf_fw_request_done(NULL, fwctx);
  
-+	wait_for_completion_timeout(fwctx->completion, msecs_to_jiffies(5000));
-+	fwctx->completion = NULL;
++	wait_for_completion_timeout(&completion, msecs_to_jiffies(5000));
 +
  	return 0;
  }