Slow down CP2102 baud rate to 921600

It seems some chip variants or drivers don't like 1M baud and only
accept 921600 baud.

README.md

@@ -127,8 +127,8 @@ the compiled `sniffle.out` binary using the UniFlash GUI.
 ## Firmware Installation (SONOFF USB Dongle)
 
 To install Sniffle on a SONOFF CC2652P dongle (equipped with a CP2102 USB/UART
-adapter), you need to use a special firmware build that uses a 1 megabit baud rate
-instead of the default 2 megabit baud rate. You can use
+adapter), you need to use a special firmware build that uses a 921600 baud rate
+(labelled 1M) instead of the default 2 megabit baud rate. You can use
 [JelmerT/cc2538-bsl](https://github.com/JelmerT/cc2538-bsl) to flash the firmware
 using the built-in ROM bootloader with the following command:
 

fw/messenger.c

@@ -14,7 +14,7 @@
 UART2_Handle uart;
 
 #ifdef UART_1M_BAUD
-static const uint32_t BAUD_RATE = 1000000;
+static const uint32_t BAUD_RATE = 921600;
 #else
 static const uint32_t BAUD_RATE = 2000000;
 #endif

python_cli/sniffle_hw.py

@@ -57,7 +57,7 @@ def __init__(self, serport=None, logger=None, timeout=None):
         if serport is None:
             serport = find_xds110_serport()
         elif is_cp2102(serport):
-            baud = 1000000
+            baud = 921600
 
         self.timeout = timeout
         self.decoder_state = SniffleDecoderState()