hw/char: Check interrupt after txctrl register is written

It's possible that the transmit watermark level (txctrl.txcnt) is updated when the user writes to txctrl register, which may decrease the transmit watermark level to less than the number of entries in the transmit FIFO. In such a case, the interrupt should be raised so we need to call sifive_uart_update_irq() to check and update interrupt when txctrl register is written. Otherwise, the interrupt will have to be delayed until next TX FIFO transmission is processed. Suggested-by: Chao Liu <chao.liu.zevorn@gmail.com> Signed-off-by: Frank Chang <frank.chang@sifive.com> Reviewed-by: Jim Shu <jim.shu@sifive.com> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Message-ID: <20260513030503.3665414-1-frank.chang@sifive.com> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>