cpu/esp32: system_get_time_ms cleanup
cpu/esp32: system_get_time_ms cleanup

There is an existing function that returns the system time in us as a 64 bit value. Converting this 64 value in us to a 32 bit value in ms is more easier and uses the complete 32 bit range. Using only the low part of the 64 bit system time in us and dividing it by 1e3 cuts the 32 bit range.
basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,miri64/RIOT,kaspar030/RIOT,basilfx/RIOT,yogo1212/RIOT,josephnoir/RIOT,josephnoir/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,yogo1212/RIOT,yogo1212/RIOT,mtausig/RIOT,ant9000/RIOT,OlegHahm/RIOT,toonst/RIOT,yogo1212/RIOT,smlng/RIOT,x3ro/RIOT,authmillenon/RIOT,jasonatran/RIOT,x3ro/RIOT,miri64/RIOT,kYc0o/RIOT,basilfx/RIOT,kaspar030/RIOT,x3ro/RIOT,yogo1212/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,toonst/RIOT,OTAkeys/RIOT,ant9000/RIOT,miri64/RIOT,toonst/RIOT,kaspar030/RIOT,authmillenon/RIOT,x3ro/RIOT,RIOT-OS/RIOT,basilfx/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,basilfx/RIOT,RIOT-OS/RIOT,smlng/RIOT,authmillenon/RIOT,toonst/RIOT,kYc0o/RIOT,smlng/RIOT,OTAkeys/RIOT,mtausig/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,josephnoir/RIOT,mtausig/RIOT,smlng/RIOT,kYc0o/RIOT,mtausig/RIOT,jasonatran/RIOT,josephnoir/RIOT,yogo1212/RIOT,smlng/RIOT,x3ro/RIOT,mtausig/RIOT,authmillenon/RIOT,miri64/RIOT