rt2x00: merge agc and vco works with link tuner

We need to perform different actions (AGC and VCO calibrations and VGC
tuning) periodically at different intervals. We don't need separate
works for those, we can use link tuner work and just check for proper
interval on it.

This fixes performing AGC and VCO calibration when scanning on STA
mode. We need to be on-channel to perform those calibrations.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>