usb: gadget: Fix a race condition in dynamic composition switching

This fix solves a race condition that causes a crash when switching
composition with USB cable disconnected.
The following race condition is fixed by protecting data->opened
with a mutex:

1. enable_store(0) is called.
2. While in enable_store(), adb_closed_callback() is invoked and
data->opened is set to false.
3. The execution is blocked on a mutex in closed_callback().
4. From adb_android_function_disable(), android_enable is invoked,
where we didn't call android_disable() just yet.

CRs-fixed: 482312
Change-Id: I7e37850a594e2e1c6d2cabbdb39760b00827afbd
Signed-off-by: Amit Blay <ablay@codeaurora.org>