usb-serial: fix oti6858.c segfault in termios handling

The oti6858 usb serial driver should use kernel_termios_to_user_termios/
user_termios_to_kernel_termios to avoid segfaults because the kernel
uses a structure differing from that of user space with a different
size.

Signed-off-by: Thomas Viehmann <tv@beamnet.de>
CC: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>