mfd: ti_am335x_tscadc: Mimic the probe from resume()

Use the number of touchscreen wires (tsc_wires) instead of the flag
indicating a touchscreen cell (tsc_cell) to mimic the logic from the
probe.

Besides keeping a certain harmony in the driver, the main benefit is
that we are going to simplify the number of entries in the main MFD
structure and tsc_cell can now be removed more easily.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-19-miquel.raynal@bootlin.com