counter: 104-quad-8: Add lock guards - filter clock prescaler

Add lock protection from race conditions to the 104-quad-8 counter
driver for filter clock prescaler code changes. Mutex calls used for
protection.
Signed-off-by: Syed Nayyar Waris <syednwaris@gmail.com>
Fixes: de65d055 ("counter: 104-quad-8: Support Filter Clock Prescaler")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/counter/104-quad-8.c

@@ -1365,6 +1365,8 @@ static ssize_t quad8_signal_fck_prescaler_write(struct counter_device *counter,
 	if (ret)
 		return ret;
 
+	mutex_lock(&priv->lock);
+
 	priv->fck_prescaler[channel_id] = prescaler;
 
 	/* Reset Byte Pointer */
@@ -1375,6 +1377,8 @@ static ssize_t quad8_signal_fck_prescaler_write(struct counter_device *counter,
 	outb(QUAD8_CTR_RLD | QUAD8_RLD_RESET_BP | QUAD8_RLD_PRESET_PSC,
 	     base_offset + 1);
 
+	mutex_unlock(&priv->lock);
+
 	return len;
 }