check msp packet rx before sending msp request

[ligenxxxx]

To auto switch baud rate(115200/230400) only when init
In the past, VTX would always send msp packets to FC after it was powered on. If the baud rates of VTX and FC did not match, the erroneous data received by FC might cause it to enter cli mode.
Now:

• VTX will not start sending msp packets from power on until it receives the correct msp data packet.
• If VTX does not receive the correct data packet for more than 2 seconds, it will switch the baud rate and continue to receive. Until the correct data packet is received, the serial port baud rate will be locked and start sending msp data packets.
• If FC changes the baud rate after the baud rate is locked, VTX will not automatically detect the baud rate again unless powered on again.

Note HDZero Race v3 VTX only supports 115200 in order to be compatible with the tramp protocol