firmware: qcom: scm: Fix tzmem state on probe retry

qcom_tzmem_enable() returns -EBUSY if called a second time, but this
causes probe retries to fail permanently if a later step in
qcom_scm_probe() defers after qcom_tzmem_enable() has already succeeded.

Use DO_ONCE() to ensure qcom_tzmem_init() runs exactly once across all
calls in a thread-safe manner. qcom_tzmem_dev is set on every call since
probe retries use the same device pointer. The result of the first
initialisation is cached and returned to every subsequent caller.

Fixes: 40289e35ca ("firmware: qcom: scm: enable the TZ mem allocator")
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260724094939.613844-4-mukesh.ojha@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/firmware/qcom/qcom_tzmem.c

@@ -15,6 +15,7 @@
 #include <linux/kernel.h>
 #include <linux/list.h>
 #include <linux/mm.h>
+#include <linux/once.h>
 #include <linux/radix-tree.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
@@ -507,14 +508,18 @@ phys_addr_t qcom_tzmem_to_phys(void *vaddr)
 }
 EXPORT_SYMBOL_GPL(qcom_tzmem_to_phys);
 
+static void qcom_tzmem_do_init(int *result)
+{
+	*result = qcom_tzmem_init();
+}
+
 int qcom_tzmem_enable(struct device *dev)
 {
-	if (qcom_tzmem_dev)
-		return -EBUSY;
+	static int result;
 
 	qcom_tzmem_dev = dev;
-
-	return qcom_tzmem_init();
+	DO_ONCE(qcom_tzmem_do_init, &result);
+	return result;
 }
 EXPORT_SYMBOL_GPL(qcom_tzmem_enable);