USB: usb-skeleton.c: fix compilation error and restored kref_put on fail in skel_open

USB: usb-skeleton.c: fix compilation error and restored kref_put on fail in skel_open

Fixing compilaton error.
Incrementing usage counter only on successful execution of skel_open.
Removing redundant locking

Some last changes in function skel_open and finally commit
71cd38d Revert "USB: usb-skeleton.c: fix open/disconnect race"
introduced a bug in function skel_open, which this patch fixes.

Changes since v2:
- refactoring
- Removing redundant mutex synchronization.

Changes since v1:
- Fixed accordingly feedback of Oliver Neukum oneukum@suse.de: also need to drop the lock.

Signed-off-by: Constantine Shulyupin <const@MakeLinux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>