dhcp: increase number of retries

Increase the number of DHCP retries from 5 to 15.

The maximum time spent performing the DHCP option is retries *
pause_between_packets.  pause_between_packets remains at 3 seconds.
Thus the maximum time spent is 45 seconds.

This helps in situations where the upstream switch has STP enabled and
is blocking the port.  Several STP implementations can block a port
for up to 30 seconds before marking it as an edge port.