cangen: use long long in time computation for -g to support >2.1s

The computation of nanosleep times for the "-g" option (delay between sends)
overflows when more than 2100ms are given.

Signed-off-by: Konrad Anton <konrad.anton@awinia.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Merge git://gitorious.org/linux-can/can-utils