platform/x86/amd/pmf: Initialize and clean up cb_mutex

`cb_mutex` was introduced in commit 9e0894d ("platform/x86/amd/pmf:
Enable Custom BIOS Inputs for PMF-TA") to prevent conccurrent access for
BIOS inputs. It however isn't initialized and so on use it may lead to
a NULL pointer dereference.

Add code to initialize on probe and clean up on destroy.

Reported-by: Yijun Shen <Yijun.Shen@dell.com>
Cc: Richard Gong <richgong@amd.com>
Fixes: 9e0894d ("platform/x86/amd/pmf: Enable Custom BIOS Inputs for PMF-TA")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-By: Yijun Shen <Yijun_Shen@Dell.com>
Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20250227170308.435862-1-superm1@kernel.org
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

drivers/platform/x86/amd/pmf/core.c

@@ -452,6 +452,7 @@ static int amd_pmf_probe(struct platform_device *pdev)
 
 	mutex_init(&dev->lock);
 	mutex_init(&dev->update_mutex);
+	mutex_init(&dev->cb_mutex);
 
 	apmf_acpi_init(dev);
 	platform_set_drvdata(pdev, dev);
@@ -477,6 +478,7 @@ static void amd_pmf_remove(struct platform_device *pdev)
 	amd_pmf_dbgfs_unregister(dev);
 	mutex_destroy(&dev->lock);
 	mutex_destroy(&dev->update_mutex);
+	mutex_destroy(&dev->cb_mutex);
 	kfree(dev->buf);
 }