clk: samsung: exynos5: Fix possible NULL pointer exception on platform_device_alloc() failure

During initialization of subdevices if platform_device_alloc() failed,
returned NULL pointer will be later dereferenced.  Add proper error
paths to exynos5_clk_register_subcmu().  The return value of this
function is still ignored because at this stage of init there is nothing
we can do.

Fixes: b06a532b ("clk: samsung: Add Exynos5 sub-CMU clock driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>