MFD: ti_tscadc: Disable TSC config registers in adc mode

AFE Pen Ctrl and TouchScreen transistors enabling is not
required when only ADC mode is being used, so check for availability of
TSC driver before accessing control register.

Signed-off-by: Patil, Rachna <rachna@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>