drivers: sensor: stm32_vref: get rid of floating point computation

Instead of using floating point operations to compute the vref voltage,
it is possible to use the sensor_value_from_milli() function.

On a STM32G0, this saves 130 bytes of flash, excluding the FP library
needed on a FPU less MCU.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>