USB: pl2303: initial TIOCGSERIAL support

I've got a trivial patch for the pl2303 driver, that's what I needed to
make the wacom serial tablet driver work properly. It uses the
TIOCGSERIAL ioctl to determine if it's talking to a serial device or
not, which I gather is rather common, but the pl2303 driver didn't
implement that ioctl.

Here's a patch, I'm not sure it's absolutely correct, I mostly looked at
other similar usbserial drivers to see what I must do, but it works for
me.

Signed-off-by: John Tsiombikas <nuclear@member.fsf.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>