staging: gasket: hold mutex on gasket driver unregistration

Take the global mutex on driver unregistration updates for proper
ordering of updates and consistent access procedures.

Signed-off-by: Zhongze Hu <frankhu@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>