sys/timex: Fix integer width issue
sys/timex: Fix integer width issue

The macros US_PER_MS and friends are assumed to be 32 bit unsigned integers
by users. However, e.g. on AVR a `1000U` is only 16 bit long. Thus, e.g.
`xtimer_usleep(100 * US_PER_MS)` will wrap around and only sleep for ~34ms.

This commit declares them as unsigned long, which is on all currently supported
platforms 32 bit wide.

kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,basilfx/RIOT,basilfx/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT