serial: dump DMM packets during stream detection

serial: dump DMM packets during stream detection

Regular operation of serial DMM drivers optionally can dump packet bytes
after the intialization phase has synchronized to the stream. Failure to
synchronize to the stream left developers without a dump, which complicates
research what went wrong.

Do dump packet content while the serial_stream_detect() routine tries to
synchronize to the stream. Use the spew level since the dump occurs upon
every attempt, which translates to: every received byte until a valid
packet was seen (or the synchronization phase expired).