When TOUCHSCREEN_ADC is enabled and IIO_BUFFER is disabled, it results
in the following Kbuild warning:

WARNING: unmet direct dependencies detected for IIO_BUFFER_CB
  Depends on [n]: IIO [=y] && IIO_BUFFER [=n]
  Selected by [y]:
  - TOUCHSCREEN_ADC [=y] && !UML && INPUT [=y] && INPUT_TOUCHSCREEN [=y] && IIO [=y]

The reason is that TOUCHSCREEN_ADC selects IIO_BUFFER_CB without depending
on or selecting IIO_BUFFER while IIO_BUFFER_CB depends on IIO_BUFFER. This
can also fail building the kernel.

Honor the kconfig dependency to remove unmet direct dependency warnings
and avoid any potential build failures.

Fixes: aa132ffb ("input: touchscreen: resistive-adc-touch: add generic resistive ADC touchscreen")
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20201102221504.541279-1-fazilyildiran@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>