ifconfig: T1793: extend set_speed_duplex() delta check
ifconfig: T1793: extend set_speed_duplex() delta check

Commit 9e4947770064 ("ifconfig: T1793: add delta check on set_speed_duplex()")
was wave1 of reducing the amount of switch-port flaps and BGP session resets.
The delta check now also handles the case of fixed speed and duplex settings.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x