SERIAL: core: add throttle/unthrottle callbacks for hardware assisted flow control

SERIAL: core: add throttle/unthrottle callbacks for hardware assisted flow control

Add two callbacks for hardware assisted flow control; we need to know
when the tty layers want us to stop and restart due to their buffer
levels.

Call a driver specific throttle/unthrottle function if and only if the
driver indicates that it is using an enabled hardware assisted flow
control method, otherwise fall back to the non-hardware assisted
methods.

Acked-by: Alan Cox <[email protected]>
Signed-off-by: Russell King <[email protected]>