x86: remove dead code in intel_clk_get_rate()

If all branches of a switch statement have a return instruction, all
subsequent lines are unreachable.

Identified with cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>