mfd: ti_am335x_tscadc: Rename a variable

We need to retrieve the number of wires used by the "secondary" device
(the touchscreen or the magnetic reader). Let's rename tsc_wires to
become tscmag_wires to clarify the fact that this variable can be used
in both situations.

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/20211015081506.933180-33-miquel.raynal@bootlin.com