usb: gadget: f_fs: Allow only one adb daemon perform device open

As part of ffs_ep0_open(), atomic variable ffs.opened is set and as part
of ffs_ep0_release() it is cleared. Also as part of release operation, in
ffs_data_clear() ffs->gadget is set to NULL.
If two adb daemons are running in parallel, then BUG ON is observed as part
of release operation as ffs->gadget is not set to NULL.

To fix the issue add check for ffs->opened to allow only one adb daemon
perform device open. This ensures open and release operation are performed
in serialized way and avoids any race.

CRs-Fixed: 730155
Change-Id: Ifccdfa6068f506bb7dfdc9945b60591da530df8f
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>