ODE: Corrected classify_ode of separable_reduced fixes #6950

In issue https://github.com/sympy/sympy/issues/6950, which is about correcting classify_ode of separable_reduced as few equations like ```sin(x)*cos(f(x)) + cos(x)*sin(f(x))*f(x).diff(x)``` and ```eq = (x**2 + f(x)**2)*f(x).diff(x) - 2*x*f(x)``` which separable_reduced classified but is giving wrong answer because it is not fitting in its general form.
This patch modifies classify_ode of separable_reduced so as not to classify those equations which are out of its scope.