ti_adc108s102: validate num_entries

num_entries can be as large as 255. However, based on the
_ti_adc108s102 implementation, num_entries must be less than
ADC108S102_CMD_BUFFER_SIZE or the cmd_buffer[] and
sampling_buffer[] arrays in the data struct could overflow.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>