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

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]

Fixes: 4472e1849db7 ("soc: xilinx: vcu: make pll post divider explicit")
Signed-off-by: Rohit Visavalia <rohit.visavalia@amd.com>
Link: https://lore.kernel.org/r/20250210113614.4149050-2-rohit.visavalia@amd.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>