Use 75% of the buffer for speeds higher than 115200

[mrkev-gh]

Higher speeds need larger buffer. Processing takes longer, so let's put the threshold at 75% of the buffer size to have some headroom. E.g. for 1Mbit with 2.5kB buffer 2048 characters would be roughly still around 20ms, and reserve additional 512 chars / 5ms to handle the buffer after the threshold is reached

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Related issue (if applicable): fixes #

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.0.241206
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I do not expect 8266 would be able to handle 1Mbit+ speeds.
To use with TCP Serial Bridge TCP_BRIDGE_BUF_SIZE needs to be increased.
Tested on ESP32-C3 and 1.5Mbit (rockchip serial console speed) with a 4kB buffer

[s-hadinger]

Looks ok to me, but note that I never use serial above 115200

[mrkev-gh]

@s-hadinger Thanks for the review. The only outstanding thing is the need to redefine the TCP_BRIDGE_BUF_SIZE. Would you be open to dynamic buffer allocation in TCP Bridge? E.g. for >115200 to allocate the size based on TCPBaudRate setting to keep 20ms worth of chars and 5ms for processing, so...assuming 8N1 that would be something like 25/(1/(baudrate/9)*1000) bytes. , for 1,5Mbit that's 4.07kB total - roughly what I'm successfully running with on ESP32. I don't have anything else to really test it on, but OTOH the current 256 byte buffer is insufficient for anything over 115200.