The value of tmnxL2tpApNasPortIdPfixString specifies
the user configurable string to be added as prefix to
the NAS-Port-Id attribute
if included in RADIUS accounting-request messages.
This field is only applicable if the 'nasPortId' bit is set in
tmnxL2tpApIncludeAttributes and tmnxL2tpApNasPortIdPfixType
is set to 'userString'.