selftests: netdevsim: Increase sleep time in hw_stats_l3.sh test

hw_stats_l3.sh test is failing often for l3 stats shows less than 20
packets after 2 seconds sleep.

This is happening since there is a race between the 2 seconds sleep and
the netdevsim actually delivering the packets.

Increase the sleep time so the packets will be delivered successfully on
time.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>