mfd: ti_am335x_tscadc: Clarify the maximum values for DT entries

Clearly define the maximum open delay and sample delay. Use these
definitions in place of a mask (which works because this is the first
field in the register) and an open-coded value. While at it reword a
little bit the error messages to make them look clearer and similar.

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/20211015081506.933180-29-miquel.raynal@bootlin.com