USB: oti6858: fix TCFLSH ioctl handling

Removes unimplemented TCFLSH handling from oti6858, because it was 
preventing TCFLSH handling by upper layer (line discipline) drivers (see 
drivers/char/tty_io.c line 3450).

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>