res_pjsip_t38.c: Fix always false if test.

Calling t38_change_state() sets the t38 state so it makes little sense to
then check the state right after the call for something else.

* Made the code in t38_interpret_parameters() reject or exit T.38 mode as
intended but not implemented.

Change-Id: Ib281263a6ed44da9448132c4e6df1e183b8a3df2