Synchronize 'wacom_wireless_work' for 2.6.38 to match 3.7

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Conditionally destroy LEDs in 'wacom_unregister_inputs'

A warning from sysfs appears after a wireless tablet with LEDs completes
a disconnect-reconnect cycle. This is caused by 'wacom_wireless_work'
calling 'wacom_unregister_inputs' for both disconnect and reconnect
events. Because 'wacom_unregister_events' unconditionally calls
'wacom_destory_leds', we end up trying to re-destroy the LEDs on
reconnect.

To prevent this, make the call to 'wacom_destory_leds' only if the input
device has already been registered. This will ensure that we only
attempt to destory the LEDs on disconnect, not on reconnect.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>