bt: PORT_WriteDataCO: fixed a bug, reduced heap memory allocation
bt: PORT_WriteDataCO: fixed a bug, reduced heap memory allocation

* a missing pointer increment caused error, if available was > 0 after the first iteration of while(available) loop.
* changed memory allocation strategy from large fixed size (RFCOMM_DATA_BUF_SIZE) to just the needed
  size, which will be below MTU

Cherry-picked from https://github.com/espressif/esp-idf/pull/1923/

espressif/esp-idf,armada-ai/esp-idf,espressif/esp-idf,mashaoze/esp-idf,www220/esp-idf,mashaoze/esp-idf,www220/esp-idf,armada-ai/esp-idf,mashaoze/esp-idf,www220/esp-idf,espressif/esp-idf,armada-ai/esp-idf,espressif/esp-idf,armada-ai/esp-idf,www220/esp-idf,www220/esp-idf,mashaoze/esp-idf,mashaoze/esp-idf