usb-serial: begin switching to ->[sg]et_serial()

add such methods for usb_serial_driver, provide tty_operations
->[sg]et_serial() calling those.  For now the lack of methods
in driver means ENOIOCTLCMD from usb-serial ->[sg]et_serial(),
making tty_ioctl() fall back to calling ->ioctl().  Once all
drivers are converted, we'll be returning -ENOTTY instead,
completing the switchover.

Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>