bluetooth: esp32: fix VHCI semaphore misuse in HCI send path

The TX flow control semaphore was initialized with a count of 1 and
released again after every bt_esp32_send() call. This broke the VHCI
flow control contract and could allow sending more packets than the
controller advertised.

This prevents host/controller overruns and ensures proper synchronization
with the ESP32 VHCI interface.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>