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.
Also add debug print for dumping the ffs gadget.

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