SerialDPM initialization fails if the first detected interface is SERDPM_UNKNOWN

In Rev. 12195 a change was made in SerailDPM_Init() which lets the detection routine fail if the detection returns 'unknown' on the first try.
This happens because a "correct" definition was used to initialize iSerDpmType (see SERDPM_UNKNOW instead of -1).

But some of the netX chips needs a DUMMY read before the SPI / SPM detection will work and with the changed
inititalization value of iSerDpmType, this 2nd detection does not happen anymore, leading into an initialization failure.