ns16550: Simplify UART and UART-interrupt probing logic.

ns16550: Simplify UART and UART-interrupt probing logic.

1. No need to check for UART existence in the polling routine. We
already check for UART existence during boot-time initialisation (see
check_existence() function).

2. No obvious need to send a dummy character. The poll routine will
run until a character is eventually sent, but for the most common use
of serial ports (console logging) that will happen almost immediately.

Signed-off-by: Keir Fraser <keir@xen.org>