clk: Fix clk_hw_get_clk() when dev is NULL

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

clk: Fix clk_hw_get_clk() when dev is NULL

Any registered clk_core structure can have a NULL pointer in its dev

field. While never actually documented, this is evidenced by the wide

usage of clk_register and clk_hw_register with a NULL device pointer,

and the fact that the core of_clk_hw_register() function also passes a

NULL device pointer.

A call to clk_hw_get_clk() on a clk_hw struct whose clk_core is in that

case will result in a NULL pointer derefence when it calls dev_name() on

that NULL device pointer.

Add a test for this case and use NULL as the dev_id if the device

pointer is NULL.