The value of tmnxSubAuthPlcyNasPortPfixString specifies the user
configurable string to be added as prefix to the NAS-Port attribute if
included in RADIUS authentication-request messages.
This field is only applicable if the 'nasPortId' bit is set in
tmnxSubAuthPlcyInclAttributes and tmnxSubAuthPlcyNasPortPfixType
is set to 'userString'.