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