wlcore: fix occasional AP TX stop after recovery

The fw_status wasn't zeroed during allocation, resulting
in uninitialized var usage, and finally causing AP
traffic stop after recovery.

The wrong value in fw_status_2->counters.tx_lnk_free_pkts
led to a bad lnk->allocated_pkts calculation in
wlcore_fw_status(), causing wl18xx_lnk_low_prio() to return
FALSE (lnk->allocated_pkts > thold).
This eventually blocked the link in wlcore_tx_work_locked(),
as wl1271_skb_dequeue() continuously returned NULL.

Fix it by zeroing wl->fw_status_1/2 during allocation.

Signed-off-by: Victor Goldenshtein <victorg@ti.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>