mfd: ti_am335x_tscadc: Gather the ctrl register logic in one place

Instead of deriving in the probe and in the resume path the value of the
ctrl register, let's do it only once in the probe, save the value of
this register (all but the subsystem enable bit) in the driver's
structure and use it from the resume callback.

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/20211015081506.933180-23-miquel.raynal@bootlin.com