USB: serial: cp210x: fix tiocmget error handling

commit de24e0a108bc48062e1c7acaa97014bce32a919f upstream.

The current tiocmget implementation would fail to report errors up the
stack and instead leaked a few bits from the stack as a mask of
modem-status flags.

Fixes: 39a66b8d ("[PATCH] USB: CP2101 Add support for flow control")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>