mfd: ti_am335x_tscadc: Unify variable naming when referencing ti_tscadc_dev

All functions in this driver reference the same ti_tscadc_dev per device
but use several different names for this structure pointer, this can
be confusing when following the code. Use the name 'tscadc' everywhere.

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