n_tty: Unthrottle tty when flushing read buffer

When the tty input buffer is full and thereby throttled,
flushing/resetting the read buffer should unthrottle to allow more
data to be received.

Signed-off-by: Karthik Manamcheri <Karthik.Manamcheri@ni.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>