Input: ti_am335x_tsc - fix STEPCONFIG setup for Z2

Input: ti_am335x_tsc - fix STEPCONFIG setup for Z2

The Z2 step configuration doesn't erase the SEL_INP_SWC_3_0 bit-field
before setting the ADC channel. This way its value could be corrupted by
the ADC channel selected for the Z1 coordinate.

Fixes: 8c896308feae ("input: ti_am335x_adc: use only FIFO0 and clean up a little")
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Link: https://lore.kernel.org/r/20211212125358.14416-3-dariobin@libero.it
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>