Bluetooth: drivers: h4: Fix uart_fifo_fill return value handling
Bluetooth: drivers: h4: Fix uart_fifo_fill return value handling

The return value of uart_fifo_fill could potentially be negative, so
make sure the code doesn't do anything bad in that case.

Fixes #39823

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr