MFC: r285840

• In mpt_send_handshake_cmd(), use bus_space_write_stream_4(9) for writing raw data to the doorbell offset in order to clarify the intent and for avoiding unnecessarily converting the endianess back and forth. Unfortunately, the same can't be done in mpt_recv_handshake_reply() as 16-bit data needs to be read using 32-bit bus accessors.
• In mpt_recv_handshake_reply(), get rid of a redundant variable.