CHROMIUM: usb: gadget: audio_source: add .free_func callback

CHROMIUM: usb: gadget: audio_source: add .free_func callback

When userspace unbinds gadget functions through configfs, the
.free_func() callback is always invoked. (in config_usb_cfg_unlink())
Implement it as a no-op to avoid the following crash:

[ 68.125679] configfs-gadget gadget: unbind function 'accessory'/ffffffc0720bf000
[ 68.133202] configfs-gadget gadget: unbind function 'audio_source'/ffffffc0012ca3c0
[ 68.142668] tegra-xudc 700d0000.usb-device: ep 0 disabled
[ 68.148186] Bad mode in Synchronous Abort handler detected, code 0x86000006
[ 68.155144] CPU: 2 PID: 1 Comm: init Tainted: G    U W 3.18.0-09419-g87296c3-dirty #561
[ 68.163743] Hardware name: Google Tegra210 Smaug Rev 1,3+ (DT)
[ 68.169566] task: ffffffc0bc8d0000 ti: ffffffc0bc8bc000 task.ti: ffffffc0bc8bc000
[ 68.177039] PC is at 0x0
[ 68.179577] LR is at usb_put_function+0x14/0x1c
....

BUG=chrome-os-partner:49140
TEST="setprop sys.usb.config accessory,audio_source" on A44 and then
switch back to default: "setprop sys.usb.config mtp,adb", no crash will
be seen.

Change-Id: I5b6141964aab861e86e3afb139ded02d4d122dab
Signed-off-by: Mark Kuo <mkuo@nvidia.com>
Reviewed-on: https://chromium-review.googlesource.com/321013
Commit-Ready: Andrew Bresticker <abrestic@chromium.org>
Tested-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>