iio: magnetometer: mmc35240: Constify struct reg_default

mmc35240_reg_defaults is not modified and can be made const to allow the
compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
   9759    3288     128   13175    3377 drivers/iio/magnetometer/mmc35240.o

After:
   text    data     bss     dec     hex filename
   9823    3224     128   13175    3377 drivers/iio/magnetometer/mmc35240.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>