asus-laptop: Fix rmmod of asus_laptop

The asus laptop driver conditionnaly registers leds in asus_led_register()
depending on their availability, but unconditionnaly unregisters them all at
exit time or when the module fails to load. Unregistering not registered leds
result in the following Oops. So we should check before unregistering.

 [<c032d2f9>] do_page_fault+0x511/0x5e9
 [<c032bae2>] error_code+0x6a/0x70
 [<c026abf8>] device_unregister+0x26/0x32
 [<f8864218>] led_classdev_unregister+0x58/0x94 [led_class]
 [<f88a90f8>] asus_led_exit+0x17/0x41 [asus_laptop]
 [<f88a91c9>] asus_laptop_exit+0xd/0x3f [asus_laptop]
 [<c013cee1>] sys_delete_module+0x17b/0x1a2
 [<c0106eae>] sysenter_past_esp+0x6b/0xa1

EIP: [<c026a9a3>] device_del+0xb/0x23a SS:ESP 0068:f594ef0c

Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: Len Brown <len.brown@intel.com>