USB: pl2303: fix error characters not being reported to ldisc

Fix regression introduced by commit
d4fc4a7b

 (tty: Fix the PL2303 private
methods for sysrq).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>