drivers: fuel_gauge: max17048: Fix overflow when calculating voltage

Annotate the calculation with type casts to force
promotion to uint32_t and then cast back down
to uint16_t for the return. This solves the issue
with invalid voltage (mV) values being returned
due to overflow during the conversion from the
register value on the max17048 chip.

Signed-off-by: Martin Calsyn <martin.calsyn@outcomex.com.au>