soc: qcom: smem: fix qcom_smem_is_available and check if __smem is valid

Commit 7a94d5f31b54 ("soc: qcom: smem: better track SMEM uninitialized
state") changed the usage of __smem and init now as an error pointer
instead of NULL.

qcom_smem_is_available() wasn't updated to reflect this change and also
.qcom_smem_remove doesn't reset it on module exit.

Update both entry to reflect new handling of __smem.

Fixes: 7a94d5f31b54 ("soc: qcom: smem: better track SMEM uninitialized state")
Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Christian Marangi <[e-mail]>
Reviewed-by: Konrad Dybcio <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/soc/qcom/smem.c

···
  */
 bool qcom_smem_is_available(void)
 {
-	return !!__smem;
+	return !IS_ERR(__smem);
 }
 EXPORT_SYMBOL_GPL(qcom_smem_is_available);
 
···
 {
 	platform_device_unregister(__smem->socinfo);
 
-	__smem = NULL;
+	/* Set to -EPROBE_DEFER to signal unprobed state */
+	__smem = ERR_PTR(-EPROBE_DEFER);
 }
 
 static const struct of_device_id qcom_smem_of_match[] = {