clk: xilinx: vcu: unregister pll_post only if registered correctly

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

clk: xilinx: vcu: unregister pll_post only if registered correctly

If registration of pll_post is failed, it will be set to NULL or ERR,

unregistering same will fail with following call trace:

Unable to handle kernel NULL pointer dereference at virtual address 008

pc : clk_hw_unregister+0xc/0x20

lr : clk_hw_unregister_fixed_factor+0x18/0x30

sp : ffff800011923850

...

Call trace:

clk_hw_unregister+0xc/0x20

clk_hw_unregister_fixed_factor+0x18/0x30

xvcu_unregister_clock_provider+0xcc/0xf4 [xlnx_vcu]

xvcu_probe+0x2bc/0x53c [xlnx_vcu]