[multi-asic] [CLI] multi_asic_namespace_validation_callback not working properly

[wenyiz2021]

Description

def multi_asic_namespace_validation_callback(ctx, param, value):
    # TODO: this condition has a bug: it should be either "multi_asic.is_multi_asic()"
    # or "self.is_multi_asic", but changing it to correct version will lead to the 
    # following cli not working on single-asic:
    # show ip(v6) bgp neighbors/network, show storm_control add/del
    # e.g. 
    # admin@str2-7050cx3-acs-02:/usr$ show ip bgp neighbor -n asic0
    # -n/--namespace is not available for single asic
    # Aborted!
    # admin@str2-7050cx3-acs-02:/usr$ show ip bgp neighbor
    # -n/--namespace is not available for single asic
    # Aborted!
    if not multi_asic.is_multi_asic:
        click.echo("-n/--namespace is not available for single asic")
        ctx.abort()
    return value

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[wenyiz2021]

This is a simple fix but require multiple UT changes

[wenyiz2021]

https://github.com/sonic-net/sonic-utilities/pull/2429