drivers: serial: pl011: fix missing TX callback with FIFO enabled

When using the interrupt UART API, it is expected that the driver will
call the callback function repeatedly while TX interrupt is enabled.
However that is not necessarily the case with the FIFO is enabled.

If the application calls uart_fifo_fill() each time with only one byte
of data, the TX interrupt will never trigger. This is because the 1/8 TX
interrupt trigger threshold is never reached. For this reason, the
callback function should be called multiple times from software as
needed.

Fixes zephyrproject-rtos/zephyr#85479

Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>