iio: adc: max11410: make vref register name arrays static const

iio: adc: max11410: make vref register name arrays static const

The vrefp_regs and vrefn_regs arrays are constant lookup tables and
are not modified.

Make them static const so they are not reinitialized on each probe
call and are placed in read-only memory. Mark the pointer array as
const as well to prevent unintended modification.

Signed-off-by: Giorgi Tchankvetadze <giorgitchankvetadze1997@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>