User input is buffered anyway, so it's pointless to set the UART_LSR_OE bit.
Instead, wait for the guest kernel to consume the current input and send a new
character when the guest is ready.

This fixes the following warnings in guest kernels:

[  207.485000] ttyS0: 1 input overrun(s)

Reported-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>