iwlwifi: memcpy from dev_cmd and not dev_cmd->hdr

Klocwork complains about copying from dev_cmd->hdr if
copying more than 4 bytes since it means part of the
copy is from the next field. This isn't a real bug,
but for not failing Klocwork next time - fix this.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>