mfd: ti_am335x_tscadc: Drop the CNTRLREG_TSC_8WIRE macro

In TI's reference manual description for the `AFE_Pen_Ctrl' bit-field
of the TSC's CTRL register, there is no mention of 8-wire touchscreens.
Even commit f0933a60

 ("mfd: ti_am335x_tscadc: Update logic in CTRL
register for 5-wire TS") says that the value of this bit-field must be
the same for 4-wire and 8-wire touchscreens. So let's remove the
CNTRLREG_TSC_8WIRE macro to avoid misunderstandings.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211125224642.21011-5-dariobin@libero.it