iwlwifi: protect SRAM debugfs

If the device is not started, we can't read its
SRAM and attempting to do so will cause issues.
Protect the debugfs read.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/dvm/debugfs.c

@@ -124,6 +124,9 @@ static ssize_t iwl_dbgfs_sram_read(struct file *file,
 	const struct fw_img *img;
 	size_t bufsz;
 
+	if (!iwl_is_ready_rf(priv))
+		return -EAGAIN;
+
 	/* default is to dump the entire data segment */
 	if (!priv->dbgfs_sram_offset && !priv->dbgfs_sram_len) {
 		priv->dbgfs_sram_offset = 0x800000;