iio: adc: spear_adc: Replace indio_dev->mlock with own device lock

As part of the general cleanup of indio_dev->mlock, this change replaces
it with a local lock on the device's state structure.

This is part of a bigger cleanup.
Link: https://lore.kernel.org/linux-iio/CA+U=[e-mail]/

Signed-off-by: Sergiu Cuciurean <[e-mail]>
Signed-off-by: Mircea Caprioru <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/spear_adc.c

···
 	struct adc_regs_spear6xx __iomem *adc_base_spear6xx;
 	struct clk *clk;
 	struct completion completion;
+	/*
+	 * Lock to protect the device state during a potential concurrent
+	 * read access from userspace. Reading a raw value requires a sequence
+	 * of register writes, then a wait for a completion callback,
+	 * and finally a register read, during which userspace could issue
+	 * another read request. This lock protects a read access from
+	 * ocurring before another one has finished.
+	 */
+	struct mutex lock;
 	u32 current_clk;
 	u32 sampling_freq;
 	u32 avg_samples;
···
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
-		mutex_lock(&indio_dev->mlock);
+		mutex_lock(&st->lock);
 
 		status = SPEAR_ADC_STATUS_CHANNEL_NUM(chan->channel) |
 			SPEAR_ADC_STATUS_AVG_SAMPLE(st->avg_samples) |
···
 		wait_for_completion(&st->completion); /* set by ISR */
 		*val = st->value;
 
-		mutex_unlock(&indio_dev->mlock);
+		mutex_unlock(&st->lock);
 
 		return IIO_VAL_INT;
 
···
 	if (mask != IIO_CHAN_INFO_SAMP_FREQ)
 		return -EINVAL;
 
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock(&st->lock);
 
 	if ((val < SPEAR_ADC_CLK_MIN) ||
 	    (val > SPEAR_ADC_CLK_MAX) ||
···
 	spear_adc_set_clk(st, val);
 
 out:
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&st->lock);
 	return ret;
 }
 
···
 	}
 
 	st = iio_priv(indio_dev);
+
+	mutex_init(&st->lock);
+
 	st->np = np;
 
 	/*