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>