iio: adc: adc12138: make array ch_to_mux static, makes object code smaller

Don't populate const array ch_to_mux on the stack, instead
make it static. Makes the object code smaller by over 200 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  12663	   1648	    128	  14439	   3867	drivers/iio/adc/ti-adc12138.o

After
   text	   data	    bss	    dec	    hex	filename
  12353	   1744	    128	  14225	   3791	drivers/iio/adc/ti-adc12138.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>