Fix show interface status with LAG (202012 branch)

[d-dashkov]

- What I did
Fixed crash of "show int status" command when there are portchannels in system.
fixes sonic-net/sonic-buildimage#6816
It`s the same fix as for master #1376

Traceback (most recent call last):
  File "/usr/local/bin/intfutil", line 521, in <module>
    main()
  File "/usr/local/bin/intfutil", line 513, in main
    interface_stat.display_intf_status()
  File "/usr/local/bin/intfutil", line 354, in display_intf_status
    self.get_intf_status()
  File "/usr/local/lib/python3.7/dist-packages/utilities_common/multi_asic.py", line 137, in wrapped_run_on_all_asics
    func(self,  *args, **kwargs)
  File "/usr/local/bin/intfutil", line 435, in get_intf_status
    self.portchannel_speed_dict = po_speed_dict(self.po_int_dict, self.db)
  File "/usr/local/bin/intfutil", line 249, in po_speed_dict
    interface_speed = '{}G'.format(interface_speed[:-3])
TypeError: 'NoneType' object is not subscriptable

- How I did it
Added check of return value from db in inftutil.
If value is None, append N/A to the output instead of formatting incorrect type.

- How to verify it

1. Create portchannel
   $ sudo config portchannel add PortChannel0011
2. Add member
   $ config portchannel mem add PortChannel0011 Ethernet80
3. Execute interface status command
   $ show int status

Signed-off-by: d-dashkov [email protected]

[d-dashkov]

@lguohan, could you review this PR? this is the same as for the master you have already merged (#1376)

[liat-grozovik]

@d-dashkov can the master fix cherry pick to 202012 or the solution is totally different?
in the case it can be cherry picked we can add a label to the original fix and ask the release maintainer to take it.

[d-dashkov]

@liat-grozovik the fix is exactly the same, it can be cherry picked

[liat-grozovik]

@d-dashkov it was taken care. Thanks @daall . You can close this PR.