USB: ftdi_sio: clean up line-status handling

Reverse priority of errors reported to ldisc so that it matches that of
other serial drivers (break takes precedence over parity, which takes
precedence over framing errors).

Also make sure overrun errors are handled as in other drivers, that is,
an overrun error is always reported and is not associated with any
received character (instead a NULL character with the TTY_OVERRUN flag
set is inserted).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>