USB: ftdi_sio: fixed handling of unsupported CSIZE setting

FTDI UARTs support only 7 or 8 data bits. Until now the ftdi_sio driver would
only report this limitation for CS6 to dmesg and fail to reflect this fact to
tcgetattr.

This patch reverts the unsupported CSIZE setting and reports the fact with less
severance to dmesg for both CS5 and CS6.

To test the patch it's sufficient to call

    stty -F /dev/ttyUSB0 cs5

which will succeed without the patch and report an error with the patch
applied.

As an additional fix this patch ensures that the control request will always
include a data bit size.

Signed-off-by: Colin Leitner <colin.leitner@gmail.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>