mfd: ti_am335x_tscadc: Fix an error message

The error message if we cannot retrieve the clock tells us that the
touchscreen controller clock was unavailable. This is wrong, this is the
"main" clock for the hardware block, it is not specific to the
touchscreen and won't change when we will introduce ADC1/magnetic reader
support so let's correct this comment.

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/20211015081506.933180-34-miquel.raynal@bootlin.com