platform/x86: intel_skl_int3472: Correct null check

platform/x86: intel_skl_int3472: Correct null check

The int3472-discrete driver can enter an error path after initialising
int3472->clock.ena_gpio, but before it has registered the clock. This will
cause a NULL pointer dereference, because clkdev_drop() is not null aware.
Instead of guarding the call to skl_int3472_unregister_clock() by checking
for .ena_gpio, check specifically for the presence of the clk_lookup, which
will guarantee clkdev_create() has already been called.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=214453
Fixes: 7540599a5ef1 ("platform/x86: intel_skl_int3472: Provide skl_int3472_unregister_clock()")
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Link: https://lore.kernel.org/r/20211008224608.415949-1-djrscally@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>