hw/char/cmsdk-apb-uart.c: Accept more input after character read

hw/char/cmsdk-apb-uart.c: Accept more input after character read

The character frontend needs to be notified that the uart receive buffer
is empty and ready to handle another character.

Previously, the uart only worked correctly when receiving one character
at a time.

Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Message-id: CAEg67GkRTw=cXei3o9hvpxG_L4zSrNzR0bFyAgny+sSEUb_kPw@mail.gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>