USB: serial: pl2303: fix line-setting error handling

Make sure to return an error on zero-length transfers when retrieving
the line settings even if the driver currently ignores the return value.

Also remove a redundant check for short transfer when setting the line
settings.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>