drivers: adc: adc_mchp_xec: Fix incorrect channel mask check

Value MCHP_ADC_MAX_CHAN_MASK defined in microchip hal as
0x07u which is different method for mask calculation then used in Zephyr
API for 8 channels (MCHP_ADC_MAX_CHAN = 8).

Calculate bitmask ourselves using BIT_MASK().

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>