mfd: ti_am335x_tscadc: Rename regmap_tscadc to regmap

The regmap structure pointer is named regmap_tscadc, this is not
consistent with other drivers and is redundant, it also contributes
to several checkpatch warnings involving long lines. Rename this.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>