regulator: max8997: Constify struct regulator_ops

These are not modified so make them const to allow the compiler to put
them in read-only memory.

Before:
   text    data     bss     dec     hex filename
  13114    8596       0   21710    54ce drivers/regulator/max8997-regulator.o

After:
   text    data     bss     dec     hex filename
  14038    7672       0   21710    54ce drivers/regulator/max8997-regulator.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200617223247.25566-6-rikard.falkeborn@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>