Staging: ipack/devices/ipoctal: fix oops when accessing "buffer"

The buffer[][] field was replaced by tty_port->xmit_buf field but there was
some places that "buffer" was still accessed, giving a kernel oops because
it was uninitialized.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>