Fixed issue where MTU not getting set for some MTUs

[mastarkey]

there is an if condition that was set for setting the expected mtu for 1500, this excluded the other two MTU sizes of 5000 and 9236 so added those two types to the if condition as well as they were failing with the got value of nil and want of expected mtu size

[OpenConfigBot]

Pull Request Functional Test Report for #3602 / 146cb40

Virtual Devices

Device	Test	Test Documentation	Job	Raw Log
Arista cEOS		RT-5.1: Singleton Interface
Cisco 8000E		RT-5.1: Singleton Interface
Cisco XRd		RT-5.1: Singleton Interface
Juniper ncPTX		RT-5.1: Singleton Interface
Nokia SR Linux		RT-5.1: Singleton Interface
Openconfig Lemming		RT-5.1: Singleton Interface

Hardware Devices

Device	Test	Test Documentation	Raw Log
Arista 7808		RT-5.1: Singleton Interface
Cisco 8808		RT-5.1: Singleton Interface
Juniper PTX10008		RT-5.1: Singleton Interface
Nokia 7250 IXR-10e		RT-5.1: Singleton Interface

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