iio: adc: ti-ads1298: add bounds check to pga_settings index

In the Linux kernel, the following vulnerability has been resolved: iio: adc: ti-ads1298: add bounds check to pga_settings index ads1298_pga_settings has 7 elements but ADS1298_MASK_CH_PGA can yield values 0-7. If it yields a value >= 7, this causes an out-of-bounds array access. Add a bounds check and return -EINVAL if the index is out of range. Note that the remaining value b111 is reserved so should not be seen in a correctly functioning system.