staging: gdm72xx: cancel work when driver unloads

cancel the work function at driver unload stage and remove
the function from the queue

Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>