mods/utime.c: avoid overrun exception in ticks_diff() 
mods/utime.c: avoid overrun exception in ticks_diff() 

Revert changes to ticks_diff() and the addition of ticks_add()
husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,husigeza/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox,Xykon/pycom-micropython-sigfox