USB: android: Fail ffs_ready (i.e. start adbd) if ADB not enabled

F_FS function notifies android composite driver of userspace client's
open and close (start/stop adbd) using android.c's ready and closed
callbacks. Typically userspace starts adbd only in ADB composition as
functionfs_bind from ready_callback happens only if ADB is enabled.
Current design has couple of issues:
1) If adbd is started before enabling ADB then USB composition gets
enabled without functionfs_bind resulting a crash in ffs_func->set_alt.
2) Additionally, even if userspace script for composition switch performs
"stop adbd" before enabling new composition, there is a possibility that
closed_callback runs in parallel with ffs_enable/disable as adb daemon
is stopped asynchronously. Even though these functions use android_dev
mutex but closed callback may not use this mutex if ready_callback
gets called before ADB/FFS is enabled, resulting in different crashes.
This is possible mainly due to above 1st issue or during quick
composition switches from ADB to non-ADB to ADB and by the time adb got
started, userspace enabled non-ADB composition.

To fix both of the above issues only option is to fail ffs_ready
callback (start adbd) if ADB is not enabled. This restriction clearly
avoid 1st issue, and for 2nd: closed callback would always use mutex
to avoid any potential synchronization issues. While we are at this,
also fix config->opened getting cleared twice from disabled_callback.

Change-Id: I9fe80d09b9eefaa87e396ff451a71026b798175b
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>