iwlwifi: protect SRAM debugfs

commit 4fc79db1 upstream.

If the device is not started, we can't read its
SRAM and attempting to do so will cause issues.
Protect the debugfs read.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Pass iwl_shared not iwl_priv pointer to iwl_is_ready_rf()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/wireless/iwlwifi/iwl-debugfs.c

@@ -232,6 +232,9 @@ static ssize_t iwl_dbgfs_sram_read(struct file *file,
 	struct iwl_priv *priv = file->private_data;
 	size_t bufsz;
 
+	if (!iwl_is_ready_rf(priv->shrd))
+		return -EAGAIN;
+
 	/* default is to dump the entire data segment */
 	if (!priv->dbgfs_sram_offset && !priv->dbgfs_sram_len) {
 		priv->dbgfs_sram_offset = 0x800000;