drivers: fuel_gauge: make read-only pointer args const in api

For the lists of properties passed as a pointer to the get_props and
set_props functions, there is no reason to not make the pointer const as
the called functions will not and should not alter the pointed-to data.

In practice, not having the pointers const can cause compilation errors
if trying to pass in a const array of properties.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@escolifesciences.com>