platform/x86: thinkpad_acpi: Use brightness_set_blocking callback for LEDs

Now a days the LED core can take care of executing brightness_set from
a workqueue if it needs to sleep, make use of this and remove a bunch
of DIY code for this.

Since this commit removes the workqueue usage for LEDs, the
led_sysfs_blink_set callback may now also sleep, this is fine.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>