speakup: switch to unsigned iterator in spk_ttyio_receive_buf2()

speakup: switch to unsigned iterator in spk_ttyio_receive_buf2()

Now, that spk_ttyio_receive_buf2() receives an unsigned count, the
iterator can/should be unsigned too. Switch to that to be explicit.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: William Hubbs <w.d.hubbs@gmail.com>
Cc: Chris Brannon <chris@the-brannons.com>
Cc: Kirk Reiser <kirk@reisers.ca>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: speakup@linux-speakup.org
Link: https://lore.kernel.org/r/20230731080244.2698-8-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>