drivers/ds18: fix `ds18_read` with negative temperatures
drivers/ds18: fix `ds18_read` with negative temperatures

When DS18 returns 0xFE6F (corresponding to -25.0625 °C), `ds18_read` returns
13877 whereas it should return -2506. This commit fixes this issue.

OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,mtausig/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,x3ro/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,ant9000/RIOT,miri64/RIOT,smlng/RIOT,yogo1212/RIOT,aeneby/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,josephnoir/RIOT,authmillenon/RIOT,toonst/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,cladmi/RIOT,mfrey/RIOT,yogo1212/RIOT,basilfx/RIOT,josephnoir/RIOT,smlng/RIOT,miri64/RIOT,authmillenon/RIOT,aeneby/RIOT,mtausig/RIOT,kYc0o/RIOT,toonst/RIOT,cladmi/RIOT,jasonatran/RIOT,mfrey/RIOT,ant9000/RIOT,OTAkeys/RIOT,mtausig/RIOT,josephnoir/RIOT,kaspar030/RIOT,mfrey/RIOT,OTAkeys/RIOT,cladmi/RIOT,miri64/RIOT,OlegHahm/RIOT,smlng/RIOT,OlegHahm/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kaspar030/RIOT,miri64/RIOT,basilfx/RIOT,toonst/RIOT,miri64/RIOT,yogo1212/RIOT,jasonatran/RIOT,mtausig/RIOT,josephnoir/RIOT,rfuentess/RIOT,kYc0o/RIOT,kaspar030/RIOT,cladmi/RIOT,aeneby/RIOT,mfrey/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,x3ro/RIOT,basilfx/RIOT,mtausig/RIOT,basilfx/RIOT,mfrey/RIOT,aeneby/RIOT,toonst/RIOT,x3ro/RIOT,cladmi/RIOT,authmillenon/RIOT,yogo1212/RIOT,rfuentess/RIOT,aeneby/RIOT,OlegHahm/RIOT,x3ro/RIOT,OlegHahm/RIOT,josephnoir/RIOT,x3ro/RIOT,kaspar030/RIOT,basilfx/RIOT,smlng/RIOT,toonst/RIOT,smlng/RIOT,rfuentess/RIOT