ospfd : Fix for ospf dead interval and hello due.

ospfd : Fix for ospf dead interval and hello due.

1. Ospf dead-interval will be set as 4 times of hello-interval, incase
if it is not set by using "ip ospf dead-interval <dead-val>".
2. On resetting hello-interval using "no ip ospf hello-interval" the
dead interval and hello due will be changed accordingly.

Signed-off-by: Kaushik <kaushik@niralnetworks.com>