USB: serial: keyspan_pda: fix information leak

USB: serial: keyspan_pda: fix information leak

The write() callback is supposed to return the number of characters
accepted or a negative errno. Since the addition of write fifo support
the keyspan_pda implementation will however return the number characters
submitted to the device if the write urb is not already in use. If this
number is larger than the number of characters passed to write(), the
line discipline continues writing data from beyond the tty write buffer.

Fix the information leak by making sure that keyspan_pda_write_start()
returns zero on success as intended.

Fixes: 034e38e8f687 ("USB: serial: keyspan_pda: add write-fifo support")
Cc: stable@vger.kernel.org	# 5.11
Signed-off-by: Johan Hovold <johan@kernel.org>