ads7846: Add disable sysfs entry / fix spin lock init, driver cleanup

Add disable attribute to support device locking mode, where a
unintentional touch event shouldn't wake up the system.

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: Juha Yrjola <juha.yrjola@nokia.com>