wlcore: consider tx_lnk_free_pkts only if valid (squash)

On recovery, fw_status might exist yet, causing
fw_status->tx_lnk_free_pkts to be NULL.

Check for it explicitly.

(this patch should be squashed into "wlcore/wl18xx/wl12xx: simplify
fw_status handling")

Signed-off-by: Eliad Peller <eliad@wizery.com>