Input: usbtouchscreen - initialize PM mutex before using it

Mutexes shall be initialized before they are used.

Fixes: 12e510db ("Input: usbtouchscreen - fix deadlock in autosuspend")
Reported-by:  <syzbot+199ea16c7f26418b4365@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>