On usb device rate configuration for sampling is performed relying on a
workqueue since it is not possible to access the device in the interrupt
context. Move the configuration of the probe_rate flag in the workqueue
in order to keep probe sampling in sync with actual rate configuration

Fixes: eb99cc95 ("mt76: mt7615: introduce mt7663u support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>