mfd: ti_am335x_tscadc: Use the new HZ_PER_MHZ macro

Before adding another frequency with even more zeroes, use the
HZ_PER_MHZ macro to clarify the number.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-26-miquel.raynal@bootlin.com

include/linux/mfd/ti_am335x_tscadc.h

@@ -9,6 +9,7 @@
 #define __LINUX_TI_AM335X_TSCADC_MFD_H
 
 #include <linux/mfd/core.h>
+#include <linux/units.h>
 
 #define REG_RAWIRQSTATUS	0x024
 #define REG_IRQSTATUS		0x028
@@ -133,7 +134,7 @@
 #define SEQ_STATUS		BIT(5)
 #define CHARGE_STEP		0x11
 
-#define ADC_CLK			3000000
+#define ADC_CLK			(3 * HZ_PER_MHZ)
 #define TOTAL_STEPS		16
 #define TOTAL_CHANNELS		8
 #define FIFO1_THRESHOLD		19