zebra: Modify way we query for inteface speed

zebra: Modify way we query for inteface speed

Initial commit of understanding interface speed changes
on startup was this commit:

dc7b3caefbd8baccb7fc3787a774e78d1a96636f

Effectively we had encountered situations on system startup
where the interface speed for a device was not properly setup
when zebra learns about the interface ( Imagine a bond being
brought up and the controlling software creating the bond
is not fast given system load, the bond's speed changes
upwards for each interface added ).

The initial workup on this was to allow a 15 second window
and then just reread the interface speed.  We've since noticed
that under heavy system load on startup this is not always sufficient.

So modify the code to wait the 15 seconds and then check the interfaces
speed.  If the interfaces speed is still MAX_UINT32T or it has changed
let's wait a bit longer and try again.

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