zebra: Limit speed lookup to at most 4 minutes

zebra: Limit speed lookup to at most 4 minutes

There exists some interface types that are slow on startup
to fully register their link speed.  Especially those that
are working with an asic backend.  The speed_update timer
associated with each interface would keep trying if the
system returned a MAX_UINT32 as the speed.  This speed
means both unknown or there is none under linux.

Since some interface types are slow on startup let's modify
FRR to try for at most 4 minutes and give up trying on those
interfaces where we never get any useful data.

Why 4 minutes?  I wanted to balance the time associated with
slow interfaces coming up with those that will never give us
a value.  So I choose 4 minutes as a good ballpark of time
to keep trying

Why not track all those interfaces and just not attempt to
do the speed lookup?  I would prefer to not keep track of these
as that I do not know all the interface types, nor do I wish
to keep programming as new ones come in.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>