regulator: max8907: Constify static structs

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
   2753    7328       0   10081    2761 drivers/regulator/max8907-regulator.o

After:
   text    data     bss     dec     hex filename
   9405     684       0   10089    2769 drivers/regulator/max8907-regulator.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200617223247.25566-5-rikard.falkeborn@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>