qemu serial: lost tx irqs (affecting FreeBSD's new uart(4) driver)

Well one problem seems to be the rx condition,
        ... if ((s->ier & UART_IER_RDI) && (s->lsr & UART_LSR_DR))
is not enough to trigger an irq, yet still causes the following
conditions not to be checked anymore at all.

Signed-off-by: Juergen Lock <nox@jelal.kn-bremen.de>
Acked-by: Jan Kiszka <jan.kiszka@web.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>