Input: imx6ul_tsc - propagate the errors

imx6ul_adc_init() may fail in two cases, so we should better
propagate the errors and make sure that the callers of
this function also check and propagate the errors accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>