USB: pl2303: fix data corruption on termios updates

commit 623c8263376c0b8a4b0c220232e7313d762cd0cc upstream.

Some PL2303 devices are known to lose bytes if you change serial
settings even to the same values as before. Avoid this by comparing the
encoded settings with the previsouly used ones before configuring the
device.

The common case was fixed by commit bf5e5834 ("pl2303: Fix mode
switching regression"), but this problem was still possible to trigger,
for instance, by using the TCSETS2-interface to repeatedly request
115201 baud, which gets mapped to 115200 and thus always triggers a
settings update.

Cc: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>