Input: stmpe-ts: preparations for STMPE ADC driver

This patch removes common ADC settings in favor to use
stmpe811_adc_common_init that is present in MFD. This is necessary in
preparation for the stmpe-adc driver, because those two drivers have
common settings for the ADC.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>