candump: fix off-by-one error in dropcount calculation

Calculations using unsigned integer types are done modulo U*INT_MAX+1,
so you get the correct difference of two values by simply subtracting.
No special handling for overflow is neccessary.

Signed-off-by: Urs Thuermann <urs@isnogud.escape.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>