Staging: quatech_usb2: Improvements to set_termios method

This patch implements several improvements to the set_termios method
suggested by Alan Cox:
* Correct check for whether any changes are being requested.
* Inform callers that mark/space parity isn't supported by always
clearing the bit.
* Pass back the actual used baud rate in case it doesn't match what was
requested.

Remaining known issues are that IXANY flow control is not supported, but
we don't tell the caller that, and input and output XON/XOFF flow
control are either both on or both off, but we don't indicate that back
to the caller either.

Signed-off-by: Richard Ash <richard@audacityteam.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>