serial: qcom-geni: do not kill the machine on fifo underrun

The Qualcomm GENI serial driver did not handle buffer flushing and used
to print discarded characters when the circular buffer was cleared.
Since commit 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
this instead resulted in a hard lockup due to
qcom_geni_serial_send_chunk_fifo() spinning indefinitely in the
interrupt handler.

The underlying bugs have now been fixed, but make sure to output NUL
characters instead of killing the machine if a similar driver bug is
ever reintroduced.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240704101805.30612-4-johan+linaro@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>