drivers: serial: ns16550: Fix TX IRQ not triggered when FIFO is empty

When uart_ns16550_irq_tx_enable() is called and the TX FIFO is already
empty, no new interrupt is generated, causing data transmission to stall
in some cases. This patch introduces a workaround to simulate an ISR
callback if the FIFO is empty when enabling the TX IRQ.

Signed-off-by: Jacky Lee <jacky.lee@egistec.com>