ieee802154: ca8210: Fix a potential UAF in ca8210_probe

In the Linux kernel, the following vulnerability has been resolved:

ieee802154: ca8210: Fix a potential UAF in ca8210_probe

If of_clk_add_provider() fails in ca8210_register_ext_clock(),

it calls clk_unregister() to release priv->clk and returns an

error. However, the caller ca8210_probe() then calls ca8210_remove(),

where priv->clk is freed again in ca8210_unregister_ext_clock(). In

this case, a use-after-free may happen in the second time we call

clk_unregister().

Fix this by removing the first clk_unregister(). Also, priv->clk could

be an error code on failure of clk_register_fixed_rate(). Use

IS_ERR_OR_NULL to catch this case in ca8210_unregister_ext_clock().