wl12xx: Acquire lock before stopping plt

__wl1271_plt_stop is called from both wl1271_plt_stop and
wl1271_unregister_hw. While wl1271_plt_stop acquires a mutex,
wl1271_unregister_hw does not.

Fix this by calling wl1271_plt_stop instead of __wl1271_plt_stop from
wl1271_unregister_hw.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>