ASoC: mchp-spdifrx: fix controls which rely on rsr register

ASoC: mchp-spdifrx: fix controls which rely on rsr register

The SPDIFRX block is clocked by 2 clocks: peripheral and generic clocks.
Peripheral clock feeds user interface (registers) and generic clock feeds
the receiver.

To enable the receiver the generic clock needs to be enabled and also the
ENABLE bit of MCHP_SPDIFRX_MR register need to be set.

The signal control exported by mchp-spdifrx driver reports wrong status
when the receiver is disabled. This can happen when requesting the signal
and the capture was not previously started. To solve this the receiver
needs to be enabled (by enabling generic clock and setting ENABLE bit of
MR register) before reading the signal status.

As with this fix there are 2 paths now that need to control the generic
clock and ENABLE bit of SPDIFRX_MR register (one path though controls, one
path though configuration) a mutex has been introduced. We can't rely on
subsystem locking as the controls are protected by
struct snd_card::controls_rwsem semaphore and configuration is protected
by a different lock (embedded in snd_pcm_stream_lock_irq()).

The introduction of mutex is also extended to other controls which rely on
SPDIFRX_RSR.ULOCK bit as it has been discovered experimentally that having
both clocks enabled but not the receiver (through ENABLE bit of SPDIFRX.MR)
leads to inconsistent values of SPDIFRX_RSR.ULOCK. Thus on some controls we
rely on software state (dev->trigger_enabled protected by mutex) to
retrieve proper values.

Fixes: ef265c55c1ac ("ASoC: mchp-spdifrx: add driver for SPDIF RX")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230130120647.638049-2-claudiu.beznea@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>