Input: ads7846 - miscellaneous fixes

- Add disable attribute to support device locking mode where
  unintentional touch event shouldn't wake up the system;
- Update comments;
- Add missing spin_lock_init;
- Do device resume with the lock held;
- Do cleanup calls / free memory in the reverse order of initialization.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>