drivers: fuel_gauge: fix integer comparison in api functions

In the fuel_gauge_get_props and fuel_gauge_set_props functions, an array
len is passed as a size_t but compared against an int, which leads to
C++ compiler warnings when using -Werror=sign-compare. The fix is to use
a size_t for the loop counter.

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