Input: imx6ul_tsc - add mask when set REG_ADC_CFG

Add mask of each function bits of REG_ADC_CFG, and clear these
function bits first, otherwise use '|=' operation may get the
wrong setting which depends on the original value of REG_ADC_CFG.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Reviewed-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>