MFD: ti_tscadc: ADC Clock check not required

ADC is ideally expected to work at a frequency of 3MHz.
The present code had a check, which returned error if the frequency
went below the threshold  value. But since AM335x supports various
working frequencies, this check is not required.
Now the code just uses the internal ADC clock divider to set the ADC
frequency w.r.t the sys clock.

Signed-off-by: Patil, Rachna <rachna@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>