usb: gadget: function: fix missing spinlock in f_uac1_legacy

commit 8778eb0927ddcd3f431805c37b78fa56481aeed9 upstream.

Add a missing spinlock protection for play_queue, because
the play_queue may be destroyed when the "playback_work"
work func and "f_audio_out_ep_complete" callback func
operate this paly_queue at the same time.

Fixes: c6994e6f ("USB: gadget: add USB Audio Gadget driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Zhang Qiang <qiang.zhang@windriver.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>